A reasonable optical system for Micro-electro-mechanical System(MEMS) based on digital sun sensor was designed.Based on scalar diffraction theory,the imaging model of the optical system for the sun sensor with a structure of circular aperture was developed.A computer simulation was carried out to obtain the parameters of optical system in the distance of 4 mm,aperture of 50 μm and the space between holes of 700 μm.The ground experiment was carried out for this optical system with a sun simulator,and the results show that the optical system is designed well and can meet the requirements of miniaturization and high accuracy of a sun sensor.