Based on the principle of reflection of polarized light and measurement of multi-angle multi-fitting algorithm, the influences of the film thickness of the material, the noise factor in actual measurement and the special angle of incidence on the measurement accuracy of film parameters are studied. Numerical simulation of SiO2 thin film sample analysis shows that the inversion error is smaller when the film thickness is greater than 150 nm, while it is large for film thickness below 50 nm; the smaller the noise the higher the measurement accuracy; there is a special angle of incidence for SiO2 films large error even wrong result will be generated if the initial angle of incidence is set at this special value. Selecting the initial angle of incidence to deviate from the special angle by ±2° can eliminate the impact of special points.