Abstract: In order to explore the characteristics of insulation aging changes of medium voltage cross-linked polyethylene cables after running for a period of time, the dielectric loss of 35 kV cable insulation with new cable and actual operating life of 1a, 3a, 5a, 7a, 10a was studied. Axial slicing was performed on the inside and outside of the cable with different operating years. A flat sample with a surface of 1 mm thickness was prepared and tested. The experimental results show that the low frequency dielectric loss factor of XLPE correlates with the aging degree. When the frequency is below 0.1 Hz, the tangent of dielectric loss angle shows a linear upward trend with the aging degree. The outer dielectric loss of the actual running cable insulation is greater than the inner side, indicating that environmental factors may be the main cause of insulation aging of the running cable. The detection of dielectric loss can evaluate the external environment of the running cable, the main insulation aging cause, and the aging degree. It is a simple and effective method suitable for practical engineering applications.