The star sensor is the most accurate attitude measure instrument on the satellite. It determines the threeaxis attitude of the satellite by imaging and recognizing stars. It is mainly consisted of optical system, electronic system and information processing system. Star sensor on the conventional satellite has large weight and volume, which is difficult to meet the mission requirements of the booming micronano satellite. It has become a major obstacle which limits the accuracy of micronano satellite positioning. Compared with the rapid development of electronic miniaturization, integration and information processing technology, the optical system has become the bottleneck of star sensor miniaturization. To solve this problem, this paper proposes a large relative aperture microsmall optical system based on aspherical 1/13, which can realize 17° full field of view with 5 lenses. MTF is better than 05 at the Nyquist frequency point. The glass combination with the same partial or close dispersion coefficient and large difference of dispersion coefficient is selected. It is effective to correct the wide spectral chromatic aberration from 500~880 nm, which can achieve full field distortion ≤0013%. This technology can help design the nanostar sensor lens with a focal length of 25 mm. Its weight is 15 g, which is only 1/5 of the domestic nanostar sensor lens. Experimental results show that the optical system can meet the requirements determined by the angular secondlevel star point center after distortion and other indicators test. It provides one kind of core guarantee for realizing the high precision nanostar sensor.