Transmission spectrum and reflectance spectrum have long been used to characterize gap semiconductor. Transmission spectrum can be measured very directly, but the influence of substrate absorption is often unavoidable. However, when using the reflectance spectrum measurement, the absorption of thin film, substrate absorption, and coherent interference will make the reflectance spectrum much more complicated. In this paper, Considering the absorption of thin film, substrate absorption, and coherent interference, we use the envelope curves algorithm to achieve the calculation formula of refractive index deduced from the reflectance spectrum. Through the analysis of the reflectance spectrum of Ga2O3film, we achieved thickness of the film, refractive index, extinction and absorption coefficient and dispersion constant.