Abstract: In view of the fracture failure of the connection bolts of diversion shell for low pressure heater drainage pump in a power plant, means such as macroscopic examination, chemical composition analysis, hardness test, metallographic examination and scanning electron microscope fracture analysis were used to analyze the fracture causes of the bolts. The results show that the fracture of the bolts was a typical toughness overload fracture. On the one hand, there were forming defects at the bottom of screw thread of the bolts, which resulted in stress concentration and reduced the bearing capacity of the bolts. On the other hand, the operating state of the drainage system was unstable and the drainage pump was subjected to a relatively large impact load. The co-action of the two above factors resulted in the overload fracture of the connection bolts at the bottom of the stress concentrated screw thread of the joint surface position. Finally, improvement suggestions were put forward.