In high-voltage power transmission systems, cable accessories serve as critical components connecting the main cables to substation equipment. Their mechanical and insulation properties significantly impact the electrical stability and operational reliability of the entire system. However, in current cable accessories, the interface between silicone rubber and cross-linked polyethylene tends to develop micro-gaps under high temperatures and stress relaxation. This leads to weak interfacial bonding and causes electric field distortion under high electric fields, thereby increasing the risk of breakdown and becoming a key weak point prone to electrical failure in cable systems. To address these issues, this study employs γ-methacryloxypropyltrimethoxysilane (KH570) to modify the surface of montmorillonite (MMT), thereby improving its compatibility with addition-cured liquid silicone rubber (LSR) matrix. Using a mechanical blending method, K-MMT/LSR composites with different doping ratios were prepared. The mechanical properties, as well as the electrical and thermal conductivity properties at different temperatures, were investigated. The results show that the insulation of the K-MMT/LSR composites improves with increasing K-MMT content. However, the mechanical properties initially increase and then decrease, indicating that the long-term reliability of these composites as cable accessory materials can be enhanced.