1. A position determining device for objects, comprising: a light emitting module; a first reflecting unit, being a long strip; a second reflecting unit, being a long strip; a mixed-type reflecting unit, comprising a stripped L-shaped reflecting mirror and a stripped retroreflector, said retroreflector being positioned above or below said reflecting mirror; wherein an approximately rectangular work area is formed to be surrounded by said mixed-type reflecting unit, said first reflecting unit and said second reflecting unit, said first and second reflecting units are respectively positioned at two side ends of said mixed-type reflecting unit, a contact of said first and second reflecting units is a corner of said work area being set as a reference point, a light beam of said light emitting module is emitted from said reference point to illuminate or scan said work area entirely; a light detecting unit, detecting a plurality of objects directly and indirectly blocking said light beam via said reflecting mirror to obtain a first signal respectively, and detecting said objects directly blocking said light beam via said retroreflector to obtain a second signal respectively, wherein said first signal comprises a plurality of first weak signals corresponding to said objects and said second signal comprises at least one second weak signal corresponding to said objects; a signal processing unit, electrically connected to said light detecting unit, processing said first signal and said second signal to obtain a plurality of included angles (A 1 ) corresponding to said first weak signals and a plurality of included angles (A 2 ) corresponding to said second weak signals, wherein the included angles (A 1 ) and (A 2 ) are formed by said blocked light beam intersecting the first reflection unit and the second reflection unit respectively; a microprocessor, electrically connected to said signal processing unit, arranging said included angles (A 1 ) with each of said included angles (A 2 ), respectively as a plurality of combinations of included angles, converting said combinations to a plurality of coordinates, comparing said coordinates with one another, selecting at least two of said coordinates with the same value among said coordinates, and confirming corresponding coordinates of said objects on said work area.