Objective To investigate the cause for cracking of hydrogen gas pipeline in order to provide reference for failure of the same type of pipes. Methods For hydrogen gas pipeline with a design pressure of 4. 8 MPa and a design temperature of 50 ℃ which was put in service in 1998, the macro morphology was observed, the mechanical properties were analyzed through tensile test and hardness test. In addition, optical microscopy analysis and scanning electron microscope (SEM) observation were conducted, and the composition of corrosion products was analyzed by Energy Dispersive Spectrometer (EDS). Results The pipe wall had no obvious corrosion thinning. The overall mechanical properties of the pipe wall conformed to the standard, the tested samples had relatively good toughness and there was no material degradation. The Matrix microstructure was normal ferrite + pearlite, and the structure distribution was even. Some martensite structure occurred in the weld area of T-branch pipe and bending pipe, which was easy to induce the occurrence of sulfide stress corrosion cracking. The internal wall of the pipe had cracks and pitting corrosion, Crack propagation was deep, with split ends, which was a typical characteristic of stress corrosion. The corrosion pits showed gathered phenomenon and had the trend of micro cracks formation. The existence of corrosion products on the internal wall of the pipe indicated that the transported medium was not pure. The corrosion products contained sulfur element, showing that the medium contained impurities such as sulfide. Conclusion The relatively high operating pressure of the pipeline, in combination with other joint action of stress and medium, led to hydrogen sulfide stress corrosion cracking of the pipeline internal wall.