Abstract: The cracking and leakage of the last stage superheater steel pipe of 1^# boiler for a new 660MW supercritical unit occur during the hydraulic test in the installation stage. Chemical composition analysis, mechanical properties at room temperature, metallographic structure observation, scanning electron microscope analysis and energy spectrum analysis were carried out for the failed steel pipe. The results show that the main reason of cracking leakage is that the inner wall of steel pipe has produced longitudinal linear defects during hot rolling and cold drawing, and the stress concentration is formed at the tip of the defect. In the process of hydraulic test, the defect tip opens to form cracks and expands, resulting in leakage of the last stage superheater.