In order to study the mechanism of axial ablation failure of cable buffer layer under operating voltage, a mathematical model of axial ablation of high voltage cables is established. According to the structure and actual size of 110 kV XLPE cable, the requirement for the separation length of the metal sheath and the outer insulation shield is calculated, and the influencing factors of the axial ablation along the surface of the high voltage cable are analyzed. The results show that the main reason for the ablation of cable buffer layer and insulation shield layer is the separation of metal sheath and cable insulation shield, and the buffer layer resistivity has a significant effect on the detachment length. Reducing the buffer layer resistivity can increase the allowable detachment length. When the resistivity is 10⁵ Ω·mm, the allowable detachment length is 1 186 mm. In addition, the allowable voltage of the buffer layer and the structure size of the buffer layer are also the characteristic parameters to control the fault of the buffer layer.