Foreign body defect such as metallic particles inside gas insulated switchgear（GIS）can cause insulationfault

so it is of great significance to detect this kind of defect. X-ray detection is an effective method to detect innerdefects in GIS

but this method is lacking in mature method of selecting parameters.In this paper

basic theory of X-ray detection is introduced

and method of selecting parameters for X-ray detection is discussed. Then proper param-eters for detecting foreign body defect for 220kV GIS is selected. Under selected parameters

unsharpness of radio-graphic imaging system is studied by using duplex wire image quality indicator

which verifies the feasibility of X-raydetection for metallic particle defects. Then

foreign body defect in GIS is detected and studied.The results show thatdefect of missing parts and copper particles defect of a diameter of 1～2 mm can be effectively detected by computed radiography system. Aluminum particles defect of 1～2 mm can be detected

but with low contrast.