To obtain the precise optical constants of thin films, LaF3 thin films of different thickness are deposited by Mo-boat evaporation on fused silica (JGS1). A model for extracting the optical constants of weak absorbing film is applied, which is based on spectrophotometry. The refractive index n and extinction coefficients k of the thin films and substrate are obtained in the range of 185~450 nm. It is found that the refractive index inhomogeneity of LaF3 thin films decreases as the film thickness becomes thick. The calculated optical performances curves fits the experimental ones well when the inhomogeneous model is employed, which can improve the precision of thin film′s optical constants determination.