Abstract: The comprehensive diagnosis method for internal discharge defects of gas insulated switchgear（ GIS） supporting insulator is studied based on ultra-high frequency（ UHF） partial discharge detection and X-ray imaging detection. Taking a110 k V GIS for example,UHF method is used to detect the partial discharge of GIS which is in operation,and after detecting the abnormal signals,the defect position and defect type are determined by interference elimination and multi-channel signal time-delay location. For the internal discharge defects of supporting insulator,the defect condition is determined by X-ray defection so as to realize the comprehensive defect diagnosis and guide the equipment maintenance.