Utilizing the concept of equivalent refractive index,the dispersive model of single SiO2 layer was presented based on the discussion of refractive index variation with the incident flux angle.The dispersion and transmittance curves of the SiO2 films were obtained.The experiment shows that it is periodic variation of refractive index along film thickness that made its equivalent refractive index related to wavelength different to the one of dense film.The larger the index variation range and its period along film thickness,the more anomalous dispersion the films performed.