Interfacial discharge between cross-linked polyethylene (XLPE) and silicone rubber (SIR) in cable accessories is a critical issue leading to insulation failure. To suppress this discharge, this study employs plasma silicon deposition using Tetraethyl Orthosilicate (TEOS) to modify the XLPE surface. The effects of deposition time on surface micro-morphology, element composition, and electrical properties were systematically investigated. Results show that a uniform silicon oxide film is formed, which significantly alters the surface roughness and trap distribution. Specifically, the sample treated for 3 min achieved the optimal state, exhibiting the lowest surface roughness (Ra = 15.4 nm) and the highest surface resistivity (857 × 1012 Ω/sq). Mechanism analysis indicates that this treatment effectively fills surface defects, thereby reducing interfacial micro-voids and increasing the contact area. Additionally, the modification facilitates surface charge dissipation by transforming deep traps into shallow traps. Consequently, the breakdown voltage of the XLPE/SIR interface increased by 66.7% compared to the untreated sample, demonstrating significant suppression of interfacial discharge.