Three different kinds of contaminated mirror exposed to indoor are obtained to study the influence of contaminated particles on mirror surface scattering in optical system. Particulate distributions of three contaminated mirrors are characterized by an electron microscope. A beam quality analyzer is used to collect the specular speckle images of clean mirror and three different contaminated mirrors and the images are analyzed in horizontal and vertical directions with bidirectional reflectance distribution function (BRDF). Research results show that the specular BRDF values of particulate-contaminated mirrors with different incident angles are in the same order of magnitude. From horizontal and vertical directions, the larger is the particle size, the more is the number of particles, the center of the specular BRDF changes slightly, the edge of them are volatile, and the mirror surface scattering will be more serious.