1. A microscopic vision measurement method based on adaptive positioning of a camera coordinate frame, comprising: Step A: acquiring pairs of synchronical images and transmitting the acquired images to a computer through an image acquisition card; Step B: calculating 3D coordinates of feature points in a scene according to the matched pairs of feature points in the scene obtained from the synchronical images and calibrated parameters of a microscopic stereo vision measurement model; and Step C: performing specific measurement according to the 3D coordinates of the feature points in the scene, and the method further comprising: calibrating the microscopic stereo vision measurement model based on adaptive positioning of the camera coordinate frame before executing the Step B, wherein the calibration comprises: Step a: setting up left and right microscopic vision measurement models by adaptively moving left and right camera coordinate frame along their own optical axes respectively, and establishing structural relation between the left and right microscopic vision measurement models; Step b: placing a planar target randomly in at least three positions in measurement space, shooting the planar target by left and right cameras to get pairs of target images, and setting up a world coordinate frame according to position of the planar target; Step c: extracting coordinates of calibration points in the target images; Step d: calibrating parameters of the left and right microscopic vision measurement models respectively; Step e: optimizing all the calibrated parameters of the microscopic stereo vision measurement model, wherein the Step a further comprises: assuming o cl is an optical center of a left microscopic vision system and a left optical axis and a left camera image plane π ul intersect at a principal point o pl , setting up a 2D left image coordinate frame o l −u l v l with a top left point on π ul as its origin, and setting up a left camera right-handed coordinate frame o dl −x dl y dl z dl with o dl in the left optical axis as its origin, o dl x dl //o l u l , o dl y dl //o l v l and o cl o dl =d l ; assuming o cr is an optical center of a right microscopic vision system and a right optical axis and a right camera image plane π ur intersect at a principal point o pr , setting up a 2D right image coordinate frame o r −u r v r with a top left point on π ur as its origin, and setting up a right camera right-handed coordinate frame o dr −x dr y dr z dr with o dr in the right optical axis as its origin, o dr x dr //o r u r , o dr y dr //o r v r and o cr o dr =d r ; and assuming o w −x w y w z w is the world coordinate frame; assuming a 3D point Q has coordinates of X w =[x w , y w , z w , 1] T in the world coordinate frame, coordinates of X dl =[x dl , y dl , z dl ] T in the left camera coordinate frame and coordinates of X dr =[x dr , y dr , z dr ] T in the right camera coordinate frame, then transformations among the coordinates are expressed as follows: X dl =R w dl X w +T w dl , X dr =R w dr X w +T w dr and X dr =RX dl +T, wherein R w dl and T w dl are a 3×3 rotation matrix and a 3×1 translation vector in the transformation from o w −x w y w z w to o dl −x dl y dl z dl , R w dr and T w dr are a 3×3 rotation matrix and a 3×1 translation vector in the transformation from o w −x w y w z w to o dr −x dr y dr z dr , R and T are a 3×3 rotation matrix and a 3×1 translation vector in the transformation from o dl −x dl y dl z dl to o dr −x dr y dr z dr , and R and T are structure parameters of the microscopic stereo vision measurement model; assuming image coordinates of left and right light path image points q l and q r of the 3D point Q are [u l ,v l ] T and [u r ,v r ] T respectively, then an ideal projection transformation of the microscopic stereo vision measurement model based on adaptive positioning of a camera coordinate frame is as follows: ξ l ⁡ [ u l v l 1 ] = [ s l ⁢ α l 0 u 0 ⁢ l / d l u 0 ⁢ ⁢ l 0 α l v 0 ⁢ l / d l v 0 ⁢ ⁢ l 0 0 1 / d l 1 ] ⁡ [ x dl y dl z dl 1 ] ; ξ r ⁡ [ u r v r 1 ] = [ s r ⁢ α r 0 u 0 ⁢ r / d r u 0 ⁢ ⁢ r 0 α r v 0 ⁢ r / d r v 0 ⁢ r 0 0 1 / d r 1 ] ⁡ [ x dr y dr z dr 1 ] where ξ l and ξ r are proportionality coefficients, s l and s r are left and right camera image scale factors respectively, [u 0l , v 0l ] T and [u 0r , v 0r ] T are coordinates of the principal points of left and right cameras respectively, d l and d r are positioning offsets of the left and right camera coordinate frames respectively, α l and α r are left and right adaptive magnifications respectively; considering primary and secondary radial distortions of lens, and assuming coordinates of actual image points q distl and q distr corresponding to ideal points q l and q r are [u distl , v distr ] T and [u distr , v distr ] T , then transformation from ideal image coordinates to actual image coordinates is as follows: { u distl = ( u l - u 0 ⁢ l ) ⁢ ( 1 + k 1 ⁢ l ⁢ r l 2 + k 2 ⁢ l ⁢ r l 4 ) + u 0 ⁢ ⁢ l v distl = ( v l - v 0 ⁢ l ) ⁢ ( 1 + k 1 ⁢ l ⁢ r l 2 + k 2 ⁢ l ⁢ r l 4 ) + v 0 ⁢ ⁢ l ; ⁢ ⁢ { u distr = ( u r - u 0 ⁢ r ) ⁢ ( 1 + k 1 ⁢ r ⁢ r r 2 + k 2 ⁢ r ⁢ r r 4 ) + u 0 ⁢ r v distr = ( v r - v 0 ⁢ r ) ⁢ ( 1 + k 1 ⁢ r ⁢ r r 2 + k 2 ⁢ r ⁢ r r 4 ) + v 0 ⁢ r where r l 2 =(u l −u 0l ) 2 +(v l −v 0l ) 2 , r r 2 =(u r −u 0r ) 2 +(v r −v 0r ) 2 , k 1l , k 2l and k 1r , k 2r are primary and secondary radial distortion coefficients of left and right cameras respectively; in summary, parameters of the established microscopic stereo vision measurement model based on adaptive positioning of the camera coordinate frame comprises: intrinsic parameters: s l , s r , [u 0l , v 0l ] T , [u 0r , v 0r ] T , d l , d r , α l , α r , k 1l , k 2l , k 1r , k 2r ; structure parameters: R, T; extrinsic parameters: R w dl , T w dl .