An inversion method for optical thin-film parameters based on reflection spectra is proposed, which does not require recording the phase and extremum of the reflection spectra. The complex refractive index and film thickness of optical thin films can be inverted using the reflectance amplitude, thus overcoming the disadvantage of existing techniques for measuring high-absorption thin films. This study analyzes reflection-spectrum data at different incident angles and uses the functional relationship among film reflectance, refractive index, and thickness to obtain the intersection points of reflection spectra in the refractive index-extinction coefficient planes at different incident angles. By integrating a global optimization algorithm, the refractive index and extinction coefficient values at various thin-film thicknesses for different wavelength nodes are solved. We identify the probability density distribution of the thin-film thickness corresponding to each wavelength node and utilize the results corresponding to the highest probability density of the most likely film thickness as the optimized thickness. Subsequently, a secondary inversion is performed to ascertain the optical parameters of the thin film. The proposed approach is applicable even when the optical characteristics of the material are unknown, as it does not necessitate fitting the material's refractive index. During the numerical-verification phase, the accuracy of the proposed method in inverting the optical parameters of high-absorption thin films is demonstrated via two computational examples. The proposed method provides a new practical tool for the development of optical thin-film technology, which is expected to promote its further application in multiple fields.