Abstract: As a pivotal component in the power supply system of high-speed trains, the cable termination undergoes significant aging and insulation degradation due to prolonged exposure to electrical and thermal stress. To solve the problem that it is difficult to effectively assess the insulation condition of the cable termination under existing conditions, we designed a combined electrical and thermal accelerated aging experiment with different aging gradients, and proposed a terahertz frequency domain characteristic to characterize the aging status of the composite insulation structure of vehicle mounted cable terminals. By conducting accelerated aging experiments, we obtained the terahertz frequency-domain parameters of the composite insulation structure of the cable termination under various aging conditions, including absorption coefficient, real part of dielectric constant, dielectric loss tangent, and refractive index. The analysis reveals distinct gradient variations in the terahertz frequency-domain parameters with aging time. Specifically, the absorption coefficient initially increases and then decreases as aging progresses, while the real part of dielectric constant, dielectric loss tangent, and refractive index all increase to various degrees. The above results indicate that terahertz frequency domain characteristics such as absorption coefficient, dielectric constant, tangent value of dielectric loss, and refractive index can be used to characterize the aging state of composite insulation structures of vehicle cable terminals under combined electric and thermal aging, and it can be used as a new method for evaluating the aging of composite insulation structures such as cable terminals.