Abstract: A gas cylinder valve suddenly fractured in normal use. The fracture causes of the cylinder valve were analyzed by means of macroscopic analysis, chemical composition analysis, metallographic examination, hardness test and scanning electron microscope analysis. The results show that there were spherical particles of lead distributed along the grain boundary in the microstructure, which weakened the grain boundary. In addition, there was low temperature medium in the bottle, and the mechanical properties of the bottle valve decreased under the low temperature environment. The transition zone between the large and small holes on the inner surface was the stress concentration part, and the crack propagated from the inside to the outside, which led to brittle fracture of the cylinder valve.