Abstract: During the hydraulic test of a high pressure boiler in a thermal power plant, the water wall tube was cracked. The cracking causes of water wall tubes were analyzed by means of macroscopic analysis, wall thickness measurement, chemical composition analysis, hardness test, metallographic examination, scanning electron microscope analysis and energy spectrum analysis. The results show that the water wall tubes were brittle cracking caused by hydrogen damage under-deposit corrosion. Hydrogen corrosion occurred in the water wall tube, and in the process of corrosion, some of the hydrogen atoms generated by steam water reaction diffused into the metal, and reacted with the carbides in pearlite to form methane. The larger methane molecules gathered in the grain boundary and caused the grain boundary to crack. Microcracks were formed along the inner wall, and the cracks growth eventually led to brittle cracking of the water wall tube.