Abstract: A water wall tube leaked of boiler of a hydropower plant. Through macro observation, chemical composition analysis, scanning electron microscopy analysis, energy spectrum analysis and microstructure observation, the reason for leakage of water wall tube was analyzed. The results showed that the reason for leakage of water wall tube was that corrosive medium in high-temperature flue gas penetrated into the wall, causing water wall tube to corrode, tube wall was thinned, and finally leaked.