The SiO2 sol were prepared by sol-gel method based on base/acid composite catalysis methods, and SiO2 thin films were deposited on K9 glass by dipping pulling method and then surface was modified by trimethylchlorosilane. The optical properties of the films were characterized by spectroscopc ellipsometer; the Cauchy model was presented in fitting spectroscopic ellipsometric data in 300-700 nm waveband for the film optical constants. At the same time the relation between membranes surface modification and optical properties was also discussed. The results show that Cauchy model could describe optical properties of SiO2 film very well in 300-650 nm waveband. The modified films have a law that with increasing the number of layers, the refractive index increases, and the maximum peak transmittance and porosity reduce, and the surface of modified film is more dense. ©, 2015, Rengong Jingti Xuebao/Journal of Synthetic Crystals. All right reserved.