The normal operation of bellows in GIS equipment is related to the safety and stability of the substations. The bellows and welds where occurs leakage in GIS equipment of a 750 kV substation are analyzed through macro-inspection,fracture analysis,micro-morphology analysis and chemical composition analysis. And the stress distribution of defective bellows was calculated by finite element method,and the failure reasons of bellows with leakage were analyzed. The results show that stress corrosion is the main reason of leakage and perforation of bellows. Meanwhile,the strength of the joint of bellows decreases due to welding defects,resulting in local crack of the weld and leakage. Therefore,in the production process of bellows,it is necessary to improve the welding procedure and strengthen the non-destructive testing of welding product quality,so as to prevent the introduction of pollutants and effectively avoid bellows failure.