Abstract: Cracking occurred in the valve leak-off connection pipe of a nuclear power unit. The cracking causes of the leak-off connection pipe were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, Vickers hardness test, scanning electron microscope and energy spectrum analysis. The results show that the cracking reason of the leak-off connection pipe was stress corrosion, and the corrosion medium was chloridion. Finally, relevant suggestions were put forward.