Specimens of PbTe single film are deposited on Ge substrates by vacuum thermal evaporation. During the temperature range of 80–300 K, the transmittance of a PbTe film within 2–15 μm is measured every 20 K by the PerkinElmer Fourier transform infrared spectroscopy cryogenic testing system. Then, the relationship between the refractive index and wavelength within 7–12 μm at different temperatures is received by the full spectrum inversion method fitting. It can be seen that the relationship conforms to the Cauchy formula, which can be fitted. Then, the relationship between the refractive index of the PbTe film and the temperature/wavelength can be expressed as n(λ,T)=5.82840 0.00304T+4.61458×10 6T2+8.00280/λ2+0.21544/λ4, which is obtained by the fitting method based on the Cauchy formula. Finally, the designed value obtained by the formula and the measured spectrum are compared to verify the accuracy of the formula.