Abstract: Objective The aim is to analyze the causes of tube bursting and leakage in the water wall during the operation of a 660 MW ultra supercritical boiler. Methods Combined with the boiler operation conditions, the cause of water wall leakage is analyzed using methods such as macroscopic analysis, chemical composition detection, microstructure analysis, obervision of fracture morphology and mechanical performance testing. Results The results show that improper welding process and operation of the butt weld of the spiral water wall tubes result in the formation of hardened microstructure and quenching cold cracks in the weld, as well as the accumulation of high levels of residual stress. During the long-term operation of the unit under the flexible working conditions of frequent large fluctuations and rapid load changes, under the combined effect of welding residual stress and the complex and high-level expansion stress of the furnace, quenching cracks of the butt weld rapidly expand and penetrate the weld, ultimately leading to leakage, and causing tubes burst and leakage when short term overheating in the outlet vertical segment of the water wall tube at the same piping system due to severe insufficient medium flow rate and heat exchange capacity. Conclusion Under flexible operating conditions, water wall leakage of 660 MW ultra supercritical boiler mostly dues to the improper welding process and operation and dramatically changes of unit load.