For the imaging quality problem which induced by the deviations from the CCD imaging target surface to the best image plane position of three linear array stereo mapping camera, the focusing precision was calculated according to the design parameters of the optical system ,and effect of the telecentric system on the mapping accuracy was analyzed. Telecentric system is adopted to meet the requirement of stereoscopic mapping , the focusing range is about ±2 mm, and the distortion can be negligible within focusing range. After focusing, the optical system imaging quality and distortion meet the requirement of high accuracy of mapping.