Abstract: The leak reason of hydrogenation pipeline made of 20 steel was analyzed by means of macroscopic and microscopic morphology analysis, penetrant testing, chemical composition analysis, microstructure analysis, mechanical property testing and EDS micro area composition analysis. The results show that the leak of the 20 steel pipeline was caused by forging crack which grew under the welding residual stress and work stress. Local corrosion at the interface of the pipeline accelerated the growth of forging cracks.