Based on the cracking of 15CrMo steel water-cooled pipe wall of the boiler in a power plant,the transverse crack in the pipe is analyzed. By macroscopic observation,scanning electron microscope,energy spectrum analysis and infrared flaw detection,the results indicate that the cracks in the coarse grain part of welding heat affected zone are reheating cracks,the residual stress and compressive stress in the interior cause the cracks to expand further,at the same time,there is a thick oxide layer to the fire surface,and the matrix structure is seriously spherical,which reduce the mechanical properties. Corrosion cracking accelerates the occurrence of pipe wall bursting in high temperature flue gas environment.