In order to improve thepositioning accuracy of mapping system,a design on optical lens of transmission semi-telecentric beam path in image space is expounded,and assembly method based on DFT(Design For Test) of single lens suit and whole optical lens are expounded.The results show that the proposed MTF of optical lens is 0.45(@77 lp/mm), the ground resolution power is 5m,and the distortion is 3/10 000.The test results indicate that each parameter can satisfy user′s qualification.