The surface flashover discharge fault of basin － type insulator during breaker operation and AC withstand voltage test is analyzed． By combing with the fault recording and SF6 gas decomposition product detection，the fault gas chamber is located accurately，the main reasons of surface flashover and degradation process are analyzed by disassembling the fault chamber． It is proposed that the processing of epoxy resin and the incomplete cleaning of insulator surface are the main causes for discharge fault． Finally，some suggestions for structure design and installation of GIS are proposed．