The stability and security of power systems are becoming increasingly prominent, which puts forward higher requirements for cable performance. The insulation layer acts as electrical insulation and core wire protection in the cable. The aging characteristics and state evaluation of insulation materials are of great significance to the safe and stable operation of the power system. In this paper, the thermal aging characteristics of XLPE and polypropylene random co-polymer (PPR) insulation of power cables are investigated. The results show that there is a critical aging time for XLPE. After the critical aging time, the macroscopic properties deteriorate sharply, and the crystallinity and peak melting temperature decrease greatly. PPR does not have a similar performance inflection point. The aging mechanism of the two materials is mainly thermal degradation. The thermal oxygen aging reaction inside the samples breaks the chemical bonds, and then generates a large number of carbonyl products. The difference in aging characteristics between PPR and XLPE is mainly due to the rubber phase structure in PP. The research in this paper can provide a theoretical basis for cable status assessment and has certain significance for maintaining the stable operation of the power system.