Abstract: Gas insulated metal enclosed switchgear (GIS) would produce discharge foreign matters in the process of production, assembly, transportation and switch action, which would reduce the insulation performance of GIS. In order to determine the source of discharge foreign matter in GIS, a method based on SEM and EDS was introduced. When the GIS was discharged, the GIS was disassembled and the discharge foreign matters were collected, and the source of the discharge foreign matters could be determined by using scanning electron microscope to observe the micro morphology and energy disperse spectrometer to analyze the components of the foreign matters. This method was simple and practical, and could be used as one of the important means to determine the source of discharge foreign matters in GIS.