Abstract: The spiral tube of the water wall of a supercritical boiler was leaked in many places. After sampling the leakage tube, the macroscopic examination, chemical composition, metallographic structure and tensile properties were analysed. Results show that, the original leakage point is located in the fusion zone of the water wall tube fin weld, which is caused by a large number of welding defects in the fin weld. The leakage of steam water blows adjacent pipe. The adjacent pipe appears multiple secondary leakage points, and the medium flow rate in adjacent pipe is reduced, resulting in short-term overheating in the downstream area of the secondary leakage points. It is recommended that the welding quality of fin welds should be strictly controlled and checked.