To investigate the ablation mechanism of the water-blocking buffer layer in high-voltage cross-linked polyethylene (XLPE) insulated cables, this study established a two-dimensional axisymmetric cable model using finite element method and electro-thermal coupling theory. The electric field and temperature distribution under varying contact conditions (dry, humid, or with high-resistance white powder) and contact defect lengths were simulated via COMSOL Multiphysics. Key results indicate that when the buffer layer tightly contacts the corrugated aluminum sheath, no significant local temperature rise occurs under normal operation. However, contact defects induce current density concentration, particularly under humid conditions. For instance, a 4-m contact defect length leads to a local temperature rise of 310.27 ℃ in the humid buffer layer, far exceeding its long-term thermal limit (90 ℃). Additionally, high-resistance white powder amplifies current density at contact points, exacerbating temperature anomalies. The study concludes that buffer layer ablation is driven by contact defects and environmental humidity. Current concentration and localized heating are intensified by moisture, accelerating material degradation. Ensuring uniform contact and controlling humidity are critical for preventing ablation. This work provides theoretical insights for optimizing high-voltage cable design and failure prevention.