Abstract: Due to the complex structure of onboard cables of high-speed electric multiple units(EMU) and potential defects during installation, partial discharges are prone to occur under the combined action of electrical, thermal, and mechanical stresses, leading to insulation aging or even failures, which seriously affect the safe operation of trains. This paper focuses on the partial discharge characteristics of internal insulation aging defects in EMU cable terminals. It first explores the insulation degradation mechanism and the evolution law of partial discharge characteristics through a combination of multi-physics field coupling simulation and experimental verification. Then, it constructs coupled models of the electric-thermal field and thermal-mechanical field of cable terminals, and analyzes the impact of typical defects on the distribution of multi-physics fields. It is found that the electric field concentrates at the junctions of the shielding layer, stress tube, and insulating tube, and temperature gradients cause differences in shrinkage between different material layers, forming air gaps that easily trigger partial discharges. Furthermore, it builds a combined aging platform of thermal cycling and power frequency voltage, and uses the high-frequency pulse current method to monitor partial discharge parameters at different aging cycles, clarifying the progressive process of defects from the formation of micro-defects, stable expansion to through-failure. Finally, based on the evolution logic of "defect expansion-discharge enhancement-insulation degradation" for internal defects in cable terminals, it effectively characterizes defect types and aging degrees through partial discharge parameters and spectrum characteristics, providing a theoretical basis and experimental evidence for the insulation state assessment and fault early warning of EMU cable terminals.