In this study, to ensure the applicability and reliability of phase-change coolants in high-frequency power electronic equipment, two kinds of environmentally friendly room-temperature phase-change coolants were taken as research objects, and a concept 80 broadband dielectric impedance spectrometer was used with a specially designed sealed test chamber to test the spectrum characteristics of the dielectric properties of the phase-change coolants. Then, the spectrum variation law was analyzed under different operating temperatures. The comparative analysis results showed that the two environmentally friendly coolants have different polarity and different temperature response law of relative permittivity. The dielectric properties of the nonpolar coolant EPRTEC#1 showed to be significantly better than those of the polar coolant EPRTEC#2 and transformer oil. Thus, coolant EPRTEC#1 can play the dual functions of insulation and cooling and can be suitable for all kinds of cooling structures and mainstream power electronic equipment. Regarding coolant EPRTEC#2, it can only be recommended for indirect cooling structures; however, its insulation performance showed to be better than that of deionized water.