Abstract: A damper extension rod on the main steam pipeline of a nuclear power plant fractured during use. The fracture causes of extension rod were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, mechanical property test and fracture analysis. The results show that the fracture mode of extension rod was fatigue fracture. The main cause of fatigue fracture was that the quality of extension rod thread was poor and it bore high stress level during operation, and the low strength also promoted the formation and expansion of fatigue cracks in extension rod to a certain extent.