Abstract: The effects of thermal aging on the physicochemical and dielectric properties of EPDM (ethylene-propylene-diene monomer) self-fusing insulation were investigated. The molecular structure, mechanical properties, microstructure, and dielectric performance of the samples before and after aging were analyzed. The results indicate the molecular chains of EPDM insulation are fractured by oxidation, resulting in the formation of a greater number of oxygen-containing groups and an increase in the carbonyl index after thermal aging. The rupture of the main molecular chains reduces the flexibility of EPDM and the gelation process among the adhesive molecules, which lead to the increase of the storage modulus and loss modulus, the decrease of the loss factor. The tensile property is decreased due to the molecular structure of the samples are destroyed during thermal aging, but the adhesive molecules in the self-fusing insulation will bond with the surface of the crosslinked polyethylene insulation to form a tight composite interface, which improves peel strength. The number of polar groups in the aged samples increase, orientation polarization is enhances and the losses increases. In the process of thermal aging, the weakening of intermolecular forces reduces the binding force on charge carriers, and the swelling effect of the adhesive molecules facilitates make it easier for charge carriers to transition and participate in conduction, resulting in an increased volumetric conductivity. Additionally, the loose disorder of the EPDM molecular structure allows for easier charge migration, increases the number of free electrons, and reduces the breakdown strength.