The stable operation of the power cable system is profoundly affected by the aging degree of the main insulation material cross-linked polyethylene (XLPE). In this paper, firstly, the accelerated aging experiments are carried out on the XLPE insulation of unused cables. Based on the changes in their macroscopic properties and microscopic structures, it is found that the antioxidant index can be used as an evaluation parameter for the insulation condition of old XLPE cables. Then, this method is applied to the aging assessment of XLPE in cables that have been in operation for 25 and 30 years. The results show that the mechanical, thermal, and electrical properties of XLPE, as well as its molecular and crystalline structure, deteriorate slowly at first and then quickly during the ageing process. The inflection point of change can be observed. The antioxidant index of XLPE decreases monotonically and continuously with the aging time. No inflection point is observed for the antioxidant index. Therefore, the antioxidant index can be used as an effective parameter for assessing the aging degree of old cables. The macroscopic performance and microstructure parameters of the 25-year-old XLPE cable are positioned before the inflection point of the accelerated aging curve of the unused XLPE cable insulation, while those of the 30-year-old XLPE cable are after the inflection point. Changes in these parameters are minimal compared to the results of testing the unused XLPE cable insulation. However, their antioxidant indices differ significantly. The internal antioxidant index of the XLPE insulation in the 25-year-old cable is far from the inflection point. And the 25-year-old cable insulation has not yet entered the stage of rapid performance deterioration. However, the internal antioxidants in the XLPE insulation of the 30-year-old cable have been exhausted. It is in the rapid deterioration stage. Therefore, its operating status requires high attention. Based on the significant findings of this study, standardized aging assessment method of actual operating cables may be established.