Abstract: A large transformer substation fire hydrant water supply pipe fractured. The causes of pipeline fracture were analyzed by macroscopic observation, chemical composition analysis, scanning electron microscopy analysis, metallographic examination, mechanical property testing and strength verification calculation. The results show that the improper rolling process of pipeline groove joints caused serious mechanical damage to the edges of the groove on the inner wall of the pipeline, disrupted the continuity of the substrate, and resulted in significant stress concentration. There was a large amount of free cementite in the pipeline organization, which made the material brittle and reduced its ability to resist rolling and cracking. The assembly of pipelines and groove joints was not standardized, resulted in a large bending moment at the edge of the clamp connecting the pipeline and the elbow. Cracks initiated and propagated at the mechanically damaged parts of the inner wall, ultimately led to pipeline fracture.