Polymer insulating materials are widely used in the field of electrical engineering, and their polarization in the electric field is an important factor affecting the insulation performance. Measuring the polarization behavior of polymer materials is of great significance for studying the electrical properties of insulating materials. In this paper, a test system was designed to measure the dielectric spectrum of polymers under high pressure conditions with pressure as a variable, and the pressure dielectric spectrum test was carried out with polypropylene (PP) material as the measured object, and the results of repeatable dielectric spectrum curves in the pressure range of 20 MPa–200 MPa were obtained, and the mean relative permittivity of PP increased from 2.47 to 2.61, with a growth rate of about 5.6%.The average molecular number density of PP in 20 MPa–200 MPa increased from 1.43 × 1028/m3 to 1.49 × 1028/m3, with a growth rate of about 4%. It is found that the distance between molecules, the increase of molecular density and the increase of electron polarizability of PP under high pressure and intensity lead to the increase of dielectric constant of polypropylene.