The very high precision star sensor is the most accurate attitude sensor at present. According to the star sensor system detection requirements, the optical system parameters are figured out. A kind of optical structures of star sensor with small F-number and good imaging quality is realized on the stage of Code V software. The system has an effective focal length of 90 mm, an F-number of 1.5, a field of view of 7°, and a working wavelength range of 0.45～0.85 μm. The imaging quality is enhanced and special requirements of the spot, energy concentration, distortion, lateral color etc., are all satisfied. The total system length is only 90.36 mm, which meets the demand of miniaturization and light weight in engineering. Meanwhile, it works steadily with high optical quality in the required temperature range and has good athermal and anti-defocus performances.