Abstract: Objective During the full power operation of a certain nuclear power unit, the welded joint of the VVP4001MP main steam inlet pipeline of the high-pressure cylinder broke and leaked, and the reasons for the welded joint fracture were studied and analyzed. Method The comprehensive analysis of fractured welded joints was conducted by macroscopical appearance inspection, chemical composition analysis, metallographic examination, mechanical property testing, scanning electron microscopy fracture analysis, installation welding document review, vibration testing, fatigue life calculation, assembly stress analysis, and pipeline structure static strength calculation. Result The results indicated that the chemical composition of the failed welded joint meeted the requirements of GB/T 14976. The average hardness of the base material was 172 HV10. The metallographic structure of the fracture area of the weld was normal, and no welding defects such as cracks or lack of fusion were found. However, there was a certain deviation between the geometric dimensions of the welded joint and the requirements of the process and standards, axial weld foot size was smaller than the minimum required by the process card, and the insertion depth of the branch pipe was smaller than the ASME standard. Fracture analysis showed that the cracks in the welded joint originated from the weld toe and propagate in a fatigue manner. The fatigue life of VVP4001MP socket welding was calculated to be 3658 d, which did not meet the requirement of 40 a of safe operation in the design. There was no excessive stress in the root weld structure. The RT film showed that there was a significant angle between the VVP4001MP main pipe branch socket and the insertion pipe axis, and based on the on-site construction logic, it was inferred that there was a strong pairing problem during the welding joint assembly. Conclusion The fracture property of VVP4001MP pipeline was high cycle fatigue fracture, with cracks initiating at the stress concentration point at the weld toe. The root cause of welded joint fracture was not only related to the inherent vibration of the pipeline, but also to newly added welding strong pairing, the combined effect of the two promoted the rapid initiation and propagation of cracks, and insufficient welding foot size was a contributing factor to fracture.