Abstract: The leak of the cover of heat exchanger was found during examination. The cracks parallel to welding line were found at the angle of welding seam of the cover. By means of macro-examination, chemical composition analysis, metallograph test, microhardness test, EDX and stress analysis, etc. to analyze the crack zone, the results showed that, it was stress corrosion cracking. The main cause to lead the cover leakage was erosion by alkali elements and Cl, S, O strong corrosion elements which are with high concentration contained in water vapor, and also the welding residual stress.