Abstract: After operation at 520 ℃ for 90000 h, the leakage of a 12Cr1MoV steam pipe butt joint occurred. After analysis through macroscopic observation, metallographic observation, SEM and EDS analysis, it is found that there is circumferential crack on one side of the weld from 3 o’clock to 9 o’clock. The fracture surface has no plastic deformation, and it completely copies the morphology of weld fusion line, and the crack surface is covered with dense high-temperature oxide.The crack propagation region is located in the fine grain heat affected zone and intercritical heat affected zone, at the same time, the microcracks and creep holes distributed along the grain boundary can be seen near the main crack, which indicates that the circumferential crack is type Ⅳ creep crack. Some displacement of the pipeline was also found at the failure site.Combined with the phenomenon that the pipe has only 180° cracked in the circumferential direction, it can be concluded that there may be abnormal bending moment at the circumferential joint during the service of the pipeline system, which is also the main driving force leading to the early type Ⅳ creep cracking of the pipeline.