Abstract: The optical degradation mechanism of ZnO powder caused by 90 keV protons is studied in this paper. The main degradation mechanism is the ionization under proton irradiation. With the proton exposure, a large number of point defects are generated on the surface of ZnO powder (VO, VO and VO), which is the main cause for the decrease of spectral reflectance. The results show that, under the proton exposures, the optical property degradation mainly occurs in the visible region, and the degradation is not serious in the infrared region.