Multipoint Cauchy method (MCM) is presented to investigate the refractive index and dispersion for each of Ge₂₀Sb₁₅Se₆₅ and Ge₂₈Sb₁₂Se₆₀ chalcogenide thin films at any wavelength in the transmission spectrum based on the regional approach method and Cauchy fitting. We theoretically calculate and compare the refractive index and dispersion curves obtained by using six different models. The results show that the most accurate results are obtained by the MCM. Two Ge—Sb—Se films are prepared by magnetron sputtering experimentally, and transmission spectrum curves are measured by Fourier infrared spectrometer, the noise is removed by segmental filtering and then the refractive index, dispersion, absorption coefficient, and optical band gap of the two films ina range of 500–2500 nm are obtained by the MCM. The results show that the refractive index of Ge₂₈Sb₁₂Se₆₀ film is larger than that of Ge₂₀Sb₁₅Se₆₅ film, which is caused by the higher polarizability and density of the former. The refractive indexes of both films decrease with wavelength increasing, so the long waves travel faster than short waves in the two films. The optical band gap of Ge₂₈Sb₁₂Se₆₀ film (1.675 eV) is smaller than that of Ge₂₀Sb₁₅Se₆₅ film (1.729 eV), and the corresponding wavelengths of the two are 740.3 nm and 717.2 nm. Finally, the microstructures of the two films are characterized by Raman spectra, and the reasons why the two chalcogenide films have different optical properties are explained from the bonding properties between the atoms.