Abstract: The water wall tube of boiler in a 200 MW coal-fired power plant leaked during the A-level maintenance hydraulic test.By means of macroscopic observation,tensile test,metallographic inspection,scanning electron microscope and energy spectrum analysis,the reasons for the leakage of water wall tube were analyzed.The results showed that due to the poor water quality of the boiler,the ineffective shutdown protection and the poor heat transfer of the pipe,the water slag rich in iron oxide and copper oxide was deposited on the inner wall of the fire surface of boiler water wall tube,resulting in under-deposit hydrogen corrosion.Caused a reduction in the strength of the pipe,which eventually led to leakage of pipe.