In order to analyze the deep cause of fault for surface fitting state between stress cone and cable body,this paper carries out cable disintegration,X-ray inspection,material inspection and simulation work for a 220 kV fault cable terminal.It is find that the interference fit on joint stress cone is higher than the installation process requirement,and the lap joint is not flat.When the stress cone and cable body surface are well bonded,the maximum electric field strength of the stress cone and the main insulation of the cable under operating voltage are much smaller than the breakdown field strength of the corresponding insulating material.When there is a small air gap between the stress cone and the cable surface,the internal electric field strength of the air gap is greater than the air breakdown field strength under the operating voltage,indicating that there is a discharge phenomenon inside the air gap under the operating voltage.Therefore,when the cable terminal stress cone,the cable surface is poorly attached,and there is a slight air gap at the interface,the main insulation damage caused by long-term discharge inside the air gap is the cause of the cable failure under the operating voltage.