Abstract: Two outer canisters of the same batch of hydraulic oil cylinder bursted successively in use. Macro and micro examination, chemical composition analysis, hardness testing and so on were carried out to analyze the bursting reason of oil cylinder. The results showed that the weld cold cracking existed on the oil cylinder. These preexisting cracks made stress concentration, so the hydrogen in material moved to the crack tip and was enriched due to stress induced diffusion . While the hydrogen content come to the up-limited of the fracture hydrogen content under work stress and welding residual stress, finally,the hydrogen-induced delayed brittle fracture occurred in the oil cylinder.