the means for achieving the aforesaid objective and the functions of the present invention will become apparent from the following description , taken in connection with the accompanying drawings , wherein a preferred embodiment of the present invention is disclosed . referring to fig1 to fig3 , a height adjusting device 1 for a bicycle seat post disclosed in the present invention includes an outer tube 10 , an inner tube 20 , an up - down adjust unit 30 , and an up - down control unit 40 . the outer tube 10 has a bottom end inserted in a seat tube of a bicycle frame so as to allow the outer tube 10 to be secured to the seat tube . an inner wall of the bottom end of the outer tube 10 is provided with a first threaded surface 11 . an outer wall of a top end of the outer tube 10 is disposed with a head end 12 . the inner tube 20 provides a bottom end inserted into an interior of the outer tube 10 from the top end of the outer tube 10 , and a top end arranged with a fixing base 21 for securing a bicycle seat . after being combined together , the outer tube 10 and the inner tube 20 form a combination . the up - down adjust unit 30 is arranged inside the combination of the outer tube 10 and the inner tube 20 . moreover , the up - down adjust unit 30 is provided with a first tube 31 and a second tube 32 inserted into the first tube 31 , wherein , the second tube 32 is shorter than the first tube 31 . an upper joining base 33 is disposed between a top - end opening of the first tube 31 and a top - end opening of the second tube 32 to combine the first tube 31 and the second tube 32 . a lower joining base 34 is fixed to a bottom - end opening of the first tube 31 . the upper joining base 33 may seal the top - end opening of the first tube 31 , and the lower joining base 34 may seal the bottom - end opening of the first tube 31 . the lower joining base 34 has an outer wall provided with a second threaded surface 341 , and the second threaded surface 341 is allowed to be screwed with the first threaded surface 11 of the outer tube 10 so as to be secured . the upper joining base 33 is arranged with an upper through hole 330 in the center thereof and the lower joining base 34 is also provided with a lower through hole 340 in the center thereof . the second tube 32 has a bottom end disposed with a valve base 35 allowing to seal a bottom - end opening of the second tube 32 . the valve base 35 provides an eccentric position arranged with a plurality of axial valve ports 350 , and a center provided with a center through hole 351 arranged axially . the up - down adjust unit 30 also includes an axle rod 36 and a firing pin 37 . the axle rod 36 has a bottom end configured with a piston 360 . the piston 360 is inside the second tube 32 . an outer wall of the piston 360 touches an inner wall of the second tube 32 . the axle rod 36 has a top end which may stay within the inner tube 20 after passing through the upper through hole 330 . moreover , the top end of the axle rod 36 is disposed with a coupling base 361 fastened with the fixing base 21 so as to allow the axle rod 36 and the inner tube 20 to axially move simultaneously . the firing pin 37 provides an top end whose outer side is arranged with an oil - block plate 370 formed by glue bonding a metal plate 370 a and a rubber plate 370 b together , as shown in fig4 and fig6 , wherein the rubber plate 370 b faces the valve port 350 . the oil - block plate 370 may stay on top of the valve base 35 after the top end of the firing pin 37 passes through the center through hole 351 of the valve base 35 , so that the oil - block plate 370 and the valve port 350 are arranged up and down . the oil - block plate 370 is large enough in diameter to cover and block the plurality of valve ports 350 . the firing pin 37 is pushed downwards by an elastic element 371 inside the valve base 35 so as to allow the oil - block plate 370 to block the valve port 350 in a normal state . the firing pin 37 has a bottom end provided with a pushing surface 372 allowing to pass through a lower through hole 340 of the lower joining base 34 to the position under the lower joining base 34 . an outer side of a middle section of the firing pin 37 within the first tube 31 is arranged with a floating piston 373 which may axially move along the firing pin 37 after being pushed . the floating piston 373 provides an outer wall touching an inner wall of the first tube 31 . a first oil chamber 374 whose interior is filled with oil is arranged inside the second tube 32 and between the piston 360 and the upper joining base 33 . a second oil chamber 375 whose interior is filled with oil is also arranged inside the second tube 32 and between the piston 360 and the valve base 35 . a third oil chamber 376 whose interior is filled with oil is arranged inside the first tube 31 and in a space formed among the valve base 35 , the floating piston 373 , and an outer wall of the second tube 32 . an air chamber 377 whose interior is filled with air is arranged inside the first tube 31 and between the floating piston 373 and the lower joining base 34 . an outer wall of a top end of the second tube 32 is provided with a plurality of oil through holes 320 to allow oil within the third oil chamber 376 to flow into the first oil chamber 374 via the oil through holes 320 when the oil inside the third oil chamber 376 is squeezed . the up - down control unit 40 , being arranged under the pushing surface 372 of the firing pin 37 , provides a control arm 41 . the control arm 41 has an end disposed with a pivot hole 42 allowing a pivot pin 43 to pass through , and an opposite end , opposite to the position of the pivot hole 42 is configured , arranged with an insertion groove 44 . wherein the pivot pin 43 is pivotally connected with a fasten hole 342 at an eccentric position of the lower joining base 34 so as to pivotally connect the control arm 41 with the lower joining base 34 . moreover , the insertion groove 44 , whose center is provided with a wire hole 440 , has an opening whose backside faces the pushing surface 372 . the outer wall of the lower joining base 34 corresponding to the wire hole 440 is configured with a concave wire - head hole 343 . the insertion groove 44 allows an end of a protecting tube 51 to insert into , wherein the protecting tube 51 is arranged outside of a controlling wire 50 and allows the controlling wire 50 to pass through . the wire hole 440 allows a first end of the controlling wire 50 to pass through . a controlling - wire head 52 of the first end of the controlling wire 50 is inserted into the wire - head hole 343 to be secured . the side of the control arm 41 facing the pushing surface 372 is arranged with a protruding push - up member 45 aligning the pushing surface 372 . when the controlling wire 50 of the bicycle handlebar is pulled , the control arm 41 is driven to take the pivot pin 43 as the pivot point to eccentrically move towards the firing pin 37 so as to direct the push - up member 45 to push the pushing surface 372 up to actuate the firing pin 37 to simultaneously axially move upwards to cause the oil - block plate 370 to move away from the valve port 350 . according to a preferred embodiment of the present invention , the aforesaid assemblies are allowed to be assembled together to form a combination shown in fig1 and fig3 . when employed , the combination of the up - down adjust unit 30 and the up - down control unit 40 can also be arranged inside the outer tube 10 and the inner tube 20 in an inverted manner without being limited by the manner employed in fig1 and fig3 . referring to fig4 , if the seat height needs to be adjusted higher , a user has to pull a second end of the controlling wire 50 at the bicycle handlebar ( the second end is opposite to the first end of the controlling wire 50 ). accordingly , the protecting tube 51 at the first end of the controlling wire 50 is pushed to direct the end of the protecting tube 51 to drive the control arm 41 to take the pivot pin 43 as the pivot point to turn pivotally to actuate the push - up member 45 to push the pushing surface 372 and the firing pin 37 up to press the elastic element 371 so as to allow the firing pin 37 to axially move upwards to cause the oil - block plate 370 to move away from the valve port 350 to enable the second oil chamber 375 and the third oil chamber 376 to communicate with each other . at this moment , don &# 39 ; t press the seat . the floating piston 373 is pushed by the air pressure within the air chamber 377 to axially move upwards to squeeze the oil inside the third oil chamber 376 to reversely flow through the valve port 350 into the second oil chamber 375 to push the piston 360 , the axle rod 36 and the inner tube 20 up . besides , the oil of the first oil chamber 374 is squeezed upwards by the piston 360 to flow into the third oil chamber 376 via the oil through hole 320 . thus , the seat can be adjusted higher . referring to fig5 , when the seat raises to a desired height , the user needs to release the controlling wire 50 . then , the end of the protecting tube 51 stops pushing the push - up member 45 of the control arm 41 so that the push - up member 45 stops pushing the firing pin 37 . consequently , the firing pin 37 is pushed back to its home position by the elastic element 371 to cause the oil - block plate 370 to block the valve port 350 to stop the oil flowing so as to locate the seat height position . if the seat height needs to be adjusted lower , the second end of the controlling wire 50 of the bicycle handlebar needs to be pulled . thus , the protecting tube 51 of the first end of the controlling wire 50 is driven to actuate the control arm 41 to take the pivot pin 43 as the pivot point to turn pivotally to direct the push - up member 45 to push the pushing surface 372 and the firing pin 37 up to axially move upwards to cause the oil - block plate 370 to move away from the valve port 350 to enable the second oil chamber 375 and the third oil chamber 376 to communicate with each other , as shown in fig6 . at this moment , the user needs to exert a force on the seat to press down the axle rod 36 and the inner tube 20 , as shown in fig7 . the oil within the second oil chamber 375 is squeezed by the piston 360 to flow through the valve port 350 into the third oil chamber 376 . with the down - press movement of the axle rod 36 and the piston 360 , part of oil inside the second oil chamber 375 may flow into the first oil chamber 374 via the oil through holes 320 . because the axle rod 36 inserts into the second tube 32 , it occupies some inner space of the first oil chamber 374 and causes the first oil chamber 374 and the second oil chamber 375 do not have enough space to accommodate the oil completely . thus , the oil flowing into the third oil chamber 376 accumulates pressure to push the floating piston 373 to move downwards to squeeze the air within the air chamber 377 . when the air pressure accumulated in the air chamber 377 is equal to the oil pressure of the third oil chamber 376 , the axle rod 36 and the inner tube 20 are not allowed to move down any more . that is the lowest position the seat can reach . after the seat is lowered to a desired height , the controlling wire 50 needs to be released . then , the firing pin 37 and the oil - block plate 370 are immediately pushed back to their home positions by the restoring force of the elastic element 371 to drive the oil - block plate 370 to block the valve port 350 . thus , the oil in the second oil chamber 375 and in the third oil chamber 376 may stop flowing so as to fix the seat height position after adjusted , as shown in fig8 . the aforesaid oil flows are all arranged and managed inside the up - down adjust unit 30 , that is , the up - down adjust unit 30 can be regarded as a closed type , so that the whole up - down adjust unit 30 is allowed to be replaced with a new one adjust unit 30 when the oil - flow control elements break down or need assembling together . therefore , it is convenient to repair and assemble . as the mentioned , the oil - block plate 370 and the valve port 350 are arranged up and down such that they may touch each other tightly in an up - down engaging manner to block the oil flows between the oil - block plate 370 of the up - down adjust unit 30 and the valve port 350 . accordingly , the oil - block plate 370 will not receive the shear stress generated from suddenly pressing down the seat when the seat height is adjusted , so as to have a long service life . the aforesaid structure is novel and requires fewer elements , so that it may reduce not only working hours but also cost in processing and assembling . it will be appreciated by those skilled in the art that variations and modifications to the invention described herein will be apparent without departing from the spirit and scope thereof . the variations and modifications as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as herein set forth .