###### Abstract
Part of the cable terminal discharge setup of the umbrella skirt sheath can be seriously damaged and even burned to the cable body of the sheath layer, which means that the discharge phenomenon on the inner surface of the insulating sheath becomes a new difficulty to solve. Aiming at this problem, related scholars have carried out a series of studies. References [8; 9] used multi-physics field simulation software to study the terminal electric field strength when there is an air gap defect inside the cable terminal, indicating that the existence of an air gap inside the terminal is one of the key factors leading to breakdown inside the terminal; Reference [10] establishes an assessment method and criterion for the severity of partial discharge in air gap defects of the cable terminals; Reference [11] proposed a classification model based on wavelet transform (WT) and the deep belief network (DBN) classification model, which provides a basis for the accurate and fast identification of corona discharges in the partial discharge signals of vehicle-mounted cable terminals. Reference [12] established a flat profile control model for cable joints and simulated the electric field distribution of cable joints with scratches in the main insulation or with air gaps in the main insulation, and the electric field distribution of joints with air gap defects, focusing on the partial discharge characteristics of joints containing air gap defects. Reference [13] used the irreversibility of solid insulation defects to propose a critical care system for tracking suspected partial discharge signals and observing the long-term accumulation of discharge spectra. The authors of Reference [14] developed a high-speed data acquisition device to obtain the raw discharge waveform data of partial discharges, which is used to suppress noise and improve the diagnostic efficiency and accuracy of the diagnostic system. The authors of Reference [15] proposed a new PD signal denoising method combining Aquila optimizer-variational mode decomposition (AO-VMD) and K-singular value decomposition (K-SVD) algorithms. The results of both demonstrate that the algorithm proposed by this paper has superior noise reduction and signal extraction performance. The authors of Reference [16] proposed a machine learning-based method for the identification and separation of partial discharge (PD) and two typical pulse interference (PI) signals from the onboard cable terminals of high-speed rolling stock. Reference [17] proposes a fifth-test method that applies a step voltage in a single test so that multiple flashovers occur in the test, thus improving the speed and accuracy of the salt-spray film test. The authors of Reference [18] studied the effect of the length and location of clean and polluted areas on the insulator's surface on the leakage current and arc length and established a theoretical model to calculate the gap impedance, gap transfer voltage, and leakage current. However, the majority of studies have focused on the individual cable terminal structure and the extraction of partial discharge signals, and there is still a lack of research on the overall cable terminal-sheath structure and the study of the complete discharge mechanism, as well as much work yet to be performed in the study of the discharge mechanism on the inner surface of the sheath. In this paper, starting from the electric field distribution characteristics of the cable terminal-sheath structure, the COMSOL simulation software establishes a geometrical model of the high-voltage terminal-insulation sheath on the roof of the vehicle, and determines the influence of the air gap, external wet dirt, and interface wet dirt on interface electric field distribution under the application of the RMS 25 kV industrial frequency voltage through simulation, finally obtaining the discharge mechanism of the inner surface of the insulating sheath of the terminal of the vehicle-mounted cable, based on the simulation results.