Abstract: The T-type bushing is a key component connecting the traction transformer and high-voltage cable in high-speed trains. To reveal the evolution of insulation performance, this study conducts electrical, thermal, and mechanical property tests on bushing materials at different aging stages. Based on the measured data, a coupled electro-thermal multi-physics simulation model is developed to analyze the influence of typical structural defects on electric and thermal field distribution, identifying local stress concentration zones and discharge-prone areas. Results show that aging and defects significantly intensify local field distortion and heat accumulation, accelerating synergistic degradation. Furthermore, a life prediction model dominated by the evolution of relative permittivity and conductivity with service mileage is established, and a failure threshold is proposed to estimate the remaining life at the fifth-level maintenance stage. The findings provide theoretical and technical support for the condition assessment, maintenance strategy, and life-cycle reliability management of T-type bushings in high-speed train applications.