Polymer-based semiconductor materials are mainly used for cable body and cable accessories to ensure long-term stability of power cables. The physical and chemical properties of the semi-conductive shielding layer of the cable will change due to aging. The new silicone material has excellent insulation and mechanical properties and is widely used in cable accessories. In this paper, the insulation materials—silicon rubber is studied. First, the silicone rubber sample is prepared. Then, the mechanical and dielectric properties of the samples before and after aging are tested. The results show that thermal aging has a significant effect on the characteristic parameters of silicone rubber. With the increase of aging time, the hardness and elastic modulus of the silicone rubber sample gradually increased, while the tensile strength and elongation at break significantly decreased with the increase of aging time. With the increase of aging time, the thermal stability of the silicone rubber sample decreases, the cross-linking degree of the system increases, and the organic property decreases. The measurement and analysis of thermal aging characteristics of silicone rubber can help to reduce the failure frequency of cable accessories and improve the safety.