Star sensor is the key instrument for high accurate satellite attitude measurement and flight control. Aiming at the special requirements of low star magnitude of threshold and large field of view, a star sensor optical system is designed, whose focal length is 55 mm, relative aperture is 1. 1 and field of view is 17°×17°. Optical passivity athermalization design is achieved through temperature property matching and optimization of optical glass material and mechanical structure material based on athermal glass map method. The stray light suppression design of two-stage shield and baffles are completed. The simulation analysis and calculation of the ghost distribution of imaging ray in view field and the stray light out of view field are completed with no-sequence ray tracing method. The results show that the RMS of the dispersion spot radius for each view field of the star sensor optical system is less than 4. 5 μm, the encircled energy within 2 × 2 pixels is larger or equal than 96% , the optical MTF in the field of view is larger than 0. 6 at cut-off frequency in the large temperature difference from -35℃ to 45℃ and the distortion is better than 0. 05% . The point source transmission ratio of stray light out of 45° avoided angle reaches 10 -8 . The radius of every order ghost spot at image plane is greater than 0. 8 mm. The optical imaging quality and stray light suppression of the star sensor can meet the requirements of high accurate attitude measurement.