Due to the excellent heat dissipation performance of GIS equipment, infrared thermometer can’t effectively detect the heat or discharge defects in GIS. Based on the analysis of the mechanism and manifestation for the internal heat generation defects in GIS, this paper put forward the corresponding diagnostic basis combining with the load information of the power grid, and use infrared and partial discharge combined detection method to analyze the heat generation defects of GIS outlet casing, it’s concluded that GIS internal contact defects, the temperature of the shell heating is not only related to the internal hot spot temperature, but also related to external temperature, gas chamber size, load size etc, the severity of the defect should be combined with the relevant factors for comprehensive analysis. The combined detection method of infrared imaging and partial discharge can effectively detect the defects of GIS discharge and locate it, which greatly improves the efficiency of fault diagnosis and ensures the safe and reliable operation of power grid equipment.