Based on p-polarized reflectance, a novel method for measuring the optical parameters of the films coated on one side of the transparent plane plate is presented. The dependence of the reflectance ratio γ on the refractive index n, extinction coefficient k and thickness h of the films is analyzed. The γ angular modulation curves of azo-dye doped PMMA films with two incident modes was experimentally measured. By data simulation and calculating model correction, the optical parameters of the films are precisely obtained. The experimental results coincide well with the theoretical analysis.