To research the reason of contacts burned of a disconnector in 252kV GIS in operation, through external measurement, disassembly inspection, theoretical calculation and other methods, it is concluded that because the outlet bushing and GIS body are not an integral foundation, the settlement of the separated foundation makes the crank arm of the disconnector offset, and the contact closing is not in place, which eventually causes the contact burning loss of the disconnector. The results show that when the center distance deviation of the connecting rod is greater than 5mm, the contact of the disconnector is in the state of rigid contact; When the center distance deviation of the connecting rod between phases is 7mm, the contact of the disconnector is in the off state.