This paper presented an optical system of star sensor. The lens system was a complicated double- Gauss configuration which had an effective focal length of 51 mm, an F-number of F/1.1, a field of view of 21°×21°(2ω=30°),and a working wavelength rang of 0.5~0.85 μm. The difference from common lens used in imaging system was that this lens system was required to have a high resolving power in the geometric pattern of stars. It was required to deliver the geometric spot dispersion in the specific rang at its focal surface and lower difference of displacements of centroids from thief ray in the interesting spectral range.