Abstract: The nano-porous SiO_(2) films with high laser damage threshold were prepared by using sol-gel process based on base and acid catalysis.Spectroscopic ellipsometer and FE-SEM were used to characterize the optical properties and surface microstructure of nano-porous SiO_2 film.The Cauchy model was presented well in fitting spectroscopic(ellipsometric) data for the film optical constants.The film optical constants in 300~700 nm waveband are obtained and the relation between microstructure and optical constants is also discussed.The difference of microstructure has no effect on the dispersion relation of the films while it has effect on the value of the optical constants. The refractive index is inversely proportional to the porosity of the film.