Abstract: The leakage reasons of the overflow tube of a starting circulating pump were studied by means of macroscopic analysis, metallographic examination, scanning electron microscope (SEM) analysis and energy dispersive spectrometer (EDS) analysis and hardness test. The results show that the failure property of the overflow tube was stress corrosion cracking caused by chloridion, and the failure was the interaction results of material quality, stress and medium. The main reason for the failure was the adhesion and enrichment of chlorides on the overflow tube dissimilar steel welding joint outer surface.