Satellite navigation urgently requires large aperture star sensor. Based on the research objective of large aperture and light weight, alarge aperture catadioptric optical system is designed using an R- C system and a spherical compensating lens.The field of view of the star sensor is 18°, the focal length is 719 mm,the diameter of the entrance pupil is 164 mm, and the working waveband is from 0.45 μm to 0.9 μm. The telecentric image plane design decreases the effect of calculating the position of centroid caused by the imaging plane defocussing and increases the irradiance uniformity. The initial parameters are calculated based on aberration theory. Then, ray tracing and optimization are performed by CODE V software.The abscured ratio is 0.317.The MTF is more than 0.81@ 26 lp/mm, which approaches the diffraction limit, 80% energy encircled is within 3×3 pixels,the lateral chromatic aberration is less than 2 μm, and the centroidal deviation is less than 2 μm.The internal baffle, external baffle, second-mirror baffle, and blocking rings are used to decrease the stray light level. The stray light of the opticalmechanical is simulated and analyzed using TracePro software,and the results show that the point source transmittance is between 10^-7 to 10^-4 in the range of the incident angle 6°~90° incident angle, which satisfies the requirements of application.