The star sensor is the key measurement equipment for autonomous attitude navigation of spacecraft. Due to the large volume and mass of the traditional star sensor, it is difficult to meet the application requirements of micro-nano satellites. The optical system is the core and bottleneck technology of star sensor miniaturization. The optical system parameter requirements of star sensor for all-sky star identification were studied and analyzed, and a micro-miniature, wide spectral band and transmission-type optical system design based on the spherical lens was proposed. The optical system has the focal length of 40 mm, the field angle of 26.4°, the relative aperture F of 2.8, and the spectral range of 450 nm～1000 nm.The volume and mass of the optical system are greatly reduced, which is suitable for the micro-nano satellites star sensor.