Abstract: In recent years, failures caused by ablation of water-blocking buffer layer in high voltage (HV) cross-linked polyethylene (XLPE) cables with corrugated aluminum sheaths have been reported frequently, and the ablation locations are mostly at the close contact positions between the buffer layer and the aluminum sheath. In order to fully understand the cause and mechanism of ablation fault and provide theoretical and data support for fault detection and prevention, this paper first analyzed the distribution of radial current in cable and the local temperature rise characteristics of buffer layer caused by uneven current through the electro-thermal field coupling simulation, and the simulated ablation tests were performed, the different phenomena of dry and damp buffer layer ablated under the applied current were observed, and the ablation mechanisms were deduced. The simulation results showed that the capacitive current was concentrated at the close contact positions between the aluminum sheath and the buffer layer, and the peak current density increased with the increase of the non-contact length, up to above 1×10³ mA/m², which could lead to a significant local temperature rise. The temperature rise was more obvious for a damp buffer layer, which can be higher than 47℃ or 155℃ when the non-contact length is 1m or 2m respectively. The characteristic temperatures at different ablation stages were measured, indicating a start point at 165℃. Finally, the heat resistance of the water-blocking buffer layer was measured and corresponds well with the values from the simulated tests, the practical possibility of local thermal ablation of the buffer layer in HV cable line was effectively verified, which was of great significance to optimize the cable structure, improve the fault prevention and detection ability, and ensure the safe and stable operation of the power grid.