There is much difference between the measurement results of polycrystalline powder pellets and single crystals. The infrared transmission spectra of single crystals can't be obtained from the powder pellets and there is an abnormal transmission peak in the infrared cut-off band on the spectra of polycrystalline powder pellets. The above phenomena are analyzed in principle. In this work, by simplifying the theoretical model of infrared dielectric properties of crystals, a method for measuring infrared dielectric properties of crystals based on transmission spectra of polycrystalline powder pellets is proposed. The infrared dielectric properties of AgGaS₂ and ZnGeP₂ crystals, such as infrared cut-off wavelength and long optical phonon transverse mode frequency, have been measured by this method. The results are in good agreement with those of single crystals. This study is of great value for screening high-quality materials in the exploration of new infrared functional crystals.