A method for evaluating insulation of a crosslinked polyethylene medium-voltage cable through polarization current includes the following steps that S1, a high-voltage end and a low-voltage end of a direct current high voltage power source (10) are directly connected with a cable core and a metal shielding layer of a crosslinked polyethylene medium-voltage cable sample (11) respectively, voltage -1kV is output, and cable polarization current is generated, wherein the sample comprises a new product and various cables at different aging degrees; S2, a weak current detection meter (12) is connected with a loop voltage end to detect the polarization current of the cable sample, the polarization current is transmitted, and polarization current data are stored through a computer (13) in real time; S3, fitting is carried out on the polarization current through an expansion Debye model to obtain model parameters, dielectric loss spectrums at the lower frequency of 0.001-0.05 Hz are obtained through Fourier transform, and the relative aging degree is evaluated by comparing the dielectric loss spectrums of the different cables. The method for evaluating insulation of the crosslinked polyethylene medium-voltage cable through the polarization current is simple and convenient in test process, free of damage and sensitive in response to partial serious defects, the overall aging degree of the XLPE cable is evaluated on the basis of the ageing mechanism, and the result is more persuasive.