Optical elements closed to photodetector always work in low temperature in infrared optical system, and the transmitted spectrum of the system would be shifted by the low temperature condition. This shift can affect the image quality of the system seriously. Some study have found that the reasons of the spectral shift are the change of the refractive index of the infrared thin-films in the system. In the paper, temperature characteristics of the infrared thin-films were studied. Based on the theory of optical thin film and analysis of several kinds of wavelength dispersion model, a new research on the temperature characteristic method of infrared thin film was put forward. With the method, the refractive index of infrared thin film at different temperature can be measured by spectral inversion according to the transmission spectra, and then, a formula for calculating refractive index of infrared thin film can be deduced based on the Cauchy dispersion model through data fitting analysis. The temperature characteristics of two kinds of infrared typical film materials(PbTe and Ge) were studied with the method, and the results verify the validity of the method.