Abstract: The optical constant (refractive index, extinction coefficient) and thickness of the film determine the optical properties of the coated part, so mastering the optical constant and thickness of the film according to the actual conditions is an important part of the film structure design and performance optimization. In this study, a Fourier transform infrared spectrometer was used to measure the reflectance spectrum curve of the sample. The target optimization function was constructed with the help of a different dispersion model, the simplex optimization algorithm was fitted to the reflectance spectrum curve, and the optical constant and thickness of the thin film were obtained using the target optimization function. The fitted optical constant and thickness of the thin film agree with the ellipsometer test results. When the inverted optical parameters and thickness of the thin film were incorporated into the theoretical reflectance calculation model and the reflectance curve obtained by the theoretical calculation model was in good agreement with the experimental test curve, the maximum relative error of the refractive index was less than 1.8%, the maximum relative error of thickness was less than 0.4%, and the maximum relative error of reflectance was less than 2%. This method only requires the measurement of the reflectance spectrum curve, and the optical parameters of the thin film can be obtained through calculations. This method has simple testing calculations, high accuracy, and a wide application range. This has important practical applications in structural design, optimization, and machining of optical thin films.