Abstract: The reasons for the leakage of the hi-pressure gas cylinder are identified by means of analyzing the fracture morphology,and the chemical composition,mechanical properties and micro-morphology of the gas cylinder body.As a result the reasons for the said leakage as revealed as follows.There are so much corrosion-caused pits on the inner surface of the cylinder bottom;the metallographic structure of the cylinder are ferrite+pearlite,and free of obvious decarbonization;the crack is originated in the said corrosion pit,and looks as brittle intergranular fracture with SCC fracture characteristics;chemical elements such as Fe,C and O are identified on the corroded spots via the energy spectrum analysis;all the above mentioned facts lead to such a conclusion that some water enters into the cylinder abnormally,and then reacts with the filled CO₂ gas to produce corrosive liquid which gets the cylinder surface corroded,and consequently forms the corrosion pits,which expand and deepen under the effect by the stress and the corrosion condition,and in turn,get the remained effective cylinder wall can not further bear the internal pressure,inevitably resulting in the leakage.