The invention provides a method for detecting XLPE insulation degree based on partial discharge frequency properties. The method includes the first step of pre-processing an XLPE cable to be detected as the following ways, the second step of taking out the preprocessed XLPE cable to be detected, enabling a high-frequency CT station to detect discharge pulse signals of the XLPE cable to be detected, and enabling a synchronous coil to detect partial PD signals of the XLPE cable to be detected, the third step of building and using an expert diagnose system to separate and classify the collected signals and recognize the discharge modes, and building characteristic spectrograms representing different types of pulse properties, and the fourth step of judging the severity of XLPE insulation defects by observing the characteristic spectrograms of the pulse properties, and using obtained partial discharge measured data to come to a conclusion through comparison. The method has the advantages that discharge signal spectrograms can be obtained by carrying out partial discharge measurement on XLPE insulation of different defect severities, and the XLPE insulation defect state can be evaluated effectively.