Abstract: Aiming at the phenomenon that the connecting bolt of damper in the main steam pipeline of a nuclear power plant was broken, a variety of physical and chemical methods were adopted to test and study the cause of fracture, and the relationship between the stress state of connecting bolt and the displacement of damper pipes was analyzed based on finite element simulation. The test and simulation results show that:the sample of the fractured connecting bolt is poor in quality, with inner hole being inconsistent with the design and composition being in segregation. The fracture of connecting bolt belongs to the fracture of overload bending, the direct reason is that the damper is subjected to the state of three-point bending, resulting in excessive stress of the connecting bolt, and eventually leading to the fracture of connecting bolt because of overload.