To explore the influence of electrothermal aging on the properties of HVAC XLPE cable during its actual operation, two circuits of 110 kV XLPE cable were subjected to a combined electrothermal accelerated aging test for 180 days. The test voltage was set at 95 kV and the test temperature was set at 90 ℃. One circuit of cable adopted constant temperature aging mode and the other circuit adopted periodic thermal cycle aging mode. The dielectric property of the cable is diagnosed by dielectric spectroscopy measurement, and the thermal property is diagnosed by thermal parameters measurement. Combined with Fourier infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) measurements, morphological changes of the insulation under different electrothermal aging modes are analyzed to construct the correlation with the dielectric and thermal properties. The results show that short-term electrothermal constant temperature aging mode effectively enhances the dielectric and thermal properties of the cable by modifying spherulites’ morphology and migrating small polar compounds; short-term electrothermal cycle aging mode also enhances the thermal properties of the cable due to the drop in small polar compounds. However, different distributions of small polar compounds and crystal interfaces in the insulation lead to extreme distortion in electric field distribution.