In order to obtain the optical constants of infrared low refractive index materials, single-layer yttrium fluoride (YF₃) and ytterbium fluoride (YbF₃) thin films were prepared on multispectral zinc sulfide substrates by electron beam thermal evaporation technique at different substrate temperatures. Spectrophotometer and Fourier transform infrared spectrometer were used to test the transmittance spectra of the optical parameters from visible to far-infrared bands, and the refractive index and extinction coefficient in the band of 0.4-14 μm were obtained by using the combination of envelope method and dispersion model fitting. The accuracy of the optical constants of YF₃ and YbF₃ films in the band of 0.4-1.6 μm was verified by the ellipsometry test results. The obtained optical constants were substituted into the TFCalc film design software, and the calculated transmittance spectrum curve of the monolayer film was in good agreement with the measured spectrum curve. The experimental results show that the optical constants obtained by this method are accurate and reliable in the ultra-wide spectral range of 0.4-14 μm.