Abstract: As common pressure vessels, the safety of oxygen cylinders is crucial. Failure analysis was performed on an exploded oxygen cylinder by analyzing the macro morphology and microstructure of the fragments, the chemical composition of the material, and the phase of the attachments. The results show that the explosion energy is far greater than that of common physical explosions. There exist some combustion characteristics on the fragments, such as melting and bluing, and there is widmannstatten structure in the microstructure. There is grease in the cylinder, and there are many fragments which present fast crack characteristics. Therefore, it can be assumed that the explosion is chemical explosion. The grease was fired by static electricity, causing the explosion.