Taking insulation breakdown of a 220 kV high － voltage cable for example，the discharge channel，installation procedure and so on are analyzed to find out the main reason of cable fault，and the grounding technique of high － voltage cable is discussed too． After dismantling the broken － down cable joint，it is found that welding technology between copper shell and aluminum sheath is not standard，and there are some phenomena such as false welding，welding off，and inadequate contact area in welding points at the two sides of cable joints． This connection between copper shell and aluminum sheath is not effective，and the abnormal discharge exists for a long time in the running． Besides，the current which is designed to flow through the copper shell，would flow through copper mesh from the outside of insulating prefabricated part，which would lead to abnormal heating at the edge of aluminum sheath where the contact resistance is larger． The long － term heating would burn the cable，and cause insulation breakdown finally．