To conform the design requirements of optical system for dual-line array stereoscopic mapping camera, an off-axis three-mirror reflective system was designed and its image quality was analyzed. The MTF of the optical system is near 0.6 (at 50 lp/mm), relative distortion is less than 3×10－4，and the calibrated absolute distortion is less than 20 μm. The optical design was optimized to meet the nominal optical performance. At the same time, in order to meet the dimensional requirements，mechanics constraints and low mass required for space application, a new layout of optical systems was adopted, which has the unique merits of miniaturization and lightening. The optical systems could be assembled in the same mechanical structure，of which the primary mirrors were assembled and aligned on the same deck with the required intersection angle，so it has advantages of highly structure stabilization and environment adaptability.