Abstract: The water cooled wall of a spiral tube cracked and leaked during the inspection of a supercritical boiler. The causes of water cooled wall cracking were comprehensively determined through on-site inspection, macroscopic analysis, microscopic analysis, chemical composition analysis, metallographic structure analysis and hardness analysis. The results show that the granular bainite has coarse pre-austenite grain boundary, and the acicular ferrite is fine in weld structure at cracking position.The main crack in the weld expands along the pre-austenite grain boundaries, showing intergranular cracking characteristics. Unreasonable component layout and poor welding quality are the root causes of weld cracking. The welding defects such as pores and slag inclusions are the source of cracks, and it leads to crack propagation under the multiple action of constraint stress, self-gravity, welding stress, boiler expansion stress and load transmitted by the tension plate. In order to prevent similar accidents, the corresponding preventive measures were proposed.