Abstract: A large number of cracks appeared at the end of the flue pipe of a pressurized steam boiler. The reasons for the cracking of the flue pipe were analyzed by macroscopic observation, scanning electron microscopy analysis, thermal calculation and structural design check. The results show that the high temperature at the end of the flue tube led to thermal fatigue cracking. There were serious design problems in the boiler structure. The length of the boiler furnace was too short, and the burner did not match the actual structure of the furnace, resulting in high temperature at the end of the combustion chamber and the flue pipe.