Pressured SF6 gas is widely used in GIS for electrical insulation as well as for arc extinction. And a chemical way for detecting the SF6 and its byproducts is a powerful diagnosis method for GIS. The present paper analyzes the decomposition of insulated gas of GIS (mostly SF6) in three cases with infrared spectrometer. As a result, it was found that the content of S2F10 can be used to decide whether spark or arc discharge leads to the fall down of GIS; CF4 can be used for judging the insulation grade of GIS. Besides, the gas leakage of the GIS should be concerned in the long lifetime of GIS. Finally, it is supposed that a data base containing the history of results from the gas diagnostics for each piece of equipment should be created for future maintenance activities.