Abstract: A heating defect of 220 kV GIS circuit-breaker is analyzed in detail. The nature of defects is judged by infrared thermography, X-ray diagnosis technology, loop resistance test and other test methods. Combined with the structural characteristics of the equipment, the defect is accurately located and the heating reason is analyzed. The diagnosis conclusion is verified by disassembly detection, which provides a scientific basis for similar fault diagnosis and analysis, and puts forward some suggestions for operation and maintenance of GIS.