Abstract: Sun sensor is an important attitude control and azimuth measurement device for spacecrafts. The operation principle of the micro sun sensor based on CMOS APS detector is introduced. The calculation method of photoelectron generated on the surface of CMOS APS is analyzed. The amount of the photoelectrons was calculated when the sunlight incidence was vertical to the surface of the earth. The composition of the optical films on the surface of the mask for the micro sun sensor was designed and the transmittance spectrum characteristic of every layer was figured out. An optical film design method of the mask for a micro sun sensor based on the CMOS APS is presented.