The spectral reflectance is measured after 60 keV proton irradiation by the spectrophotometer. The microscopic mechanism of irradiation damage of Al film reflector is studied by the slow positron annihilation technique and X-ray diffraction. The results show that the spectral reflectance of Al film reflectors decreases in the wavelength range of 200—800nm with increasing proton irradiation fluence and the radiation damage is mainly concentrated on the Al film of the reflector. The implanted protons may fill the dcfccts in Al film layer, thus decreases the electron density of Al film and enhances the interband transition of loose bound electrons. The interband transition can be excited by electromagnetic waves ranging from UV to visible and the spectral reflectance decreases in the corresponding wave band, which eventually results of the degradation of optical property of the reflector.