Abstract: During the routine inspection of major maintenance in a nuclear power plant, it was discovered that the flange bolts of outer heat shield and inner sleeve of the main pump cooling system had fractured during operation. Macroscopic observation, chemical composition analysis, vickers hardness test, metallographic inspection, and energy spectrum analysis were conducted on the fractured flange bolts. The reasons of the fracture were analysed in combination with the operating environment of main pump flange bolts. Results show that, the failure mode of the bolts is low-cycle fatigue fracture. In normal operation, temperature cycle changes and mechanical constraints form the two necessary conditions for fatigue fracture failure of bolts. In order to ensure the reliable operation of the main pump cooling system in the nuclear power plant, it is recommended to shorten the inspection cycle of bolts from 5 years to 2-3 years.