Uniformity of thin-film thickness is one of the main standards of judging optical thin film. The bad uniformity of thin film can damage the characteristics of costing system. As a main material of low refractive index to preparation of optic thin film,SiO₂ is poor thermal conductivity and sublimes. The particularity of evaporation characteristic of SiO₂ leads to obvious change of thin film thickness when SiO₂ is evaporated by e-beam method,which affects the quality of thin film heavily. For the purpose of analyzing the impacts of evaporation characteristics of SiO₂ on uniformity of thin film thickness,the heat distribution of e-beam spot is calculated and simulated,and the mass distribution is calculated. According to the mass distribution and the change of evaporation angle of point on evaporation surface,the thin film thickness distribution on spherical surface and flat surface is calculated. The reason of making the impact of evaporation characteristics of SiO₂ on uniformity of thin film thickness,and the particularity of thin film thickness distribution are concluded,which offer references of adjusting and improving the technological parameters for thin film coating of SiO₂ material.