The properties of optical system are affected by laser scattering characteristics of optical mirror significantly. Bidirectional reflectance distribution function (BRDF) of the optical mirror is simulated and calculated by modified Mie theory. The results show that the scattering characteristics of light are affected by particulate contaminants on the surface of a mirror obviously. In addition, the wavelength of a beam of incident laser is inversely proportional to BRDF value and scattering will becomes very complicated as the wavelength is shorter. Keeping the surface of optical mirror clean is very important for maintaining the properties of optical mirror.