A minitype optical system used in stereo camera was designed based on civilian low cost stereo imaging requirement, the optical system aimed to achieve 50 m ground sampling distance at 250 km orbit altitude. This optical system can both achieve stereo imaging function and large format array imaging function combined with a CMOS. The system has a focal length of 32 mm, a field of view of 66°, an F-number of 6.8 and a working wavelength range of 450-750 nm. The optical system consists of two parts of optical configurations-retrofocus lens and apochromatic microscope. The system design value of MTF achieves 0.49@78 lp/mm, distortion value less than 0.1%, the relative illumination of marginal field of view achieves 0.83, principal ray incoming angle in image plane of marginal field less than 15°. The testing result after optical mounting centering alignment shows that the MTF of each field of view all achieve 0.4@80 lp/mm, which meets the static MTF requirement in laboratory. This optical system has better performances in imaging width and distortion value compared with other international famous stereo cameras.