Abstract: The boiler should have qualified sealing performance during service, otherwise its working efficiency and safety will be affected. This work is based on the background of gas-fired steam water boiler failure caused by lots of cracks in the welding area between the tube bundle and drum. A comprehensive analysis was conducted on the reasons for boiler failure, considering various aspects including the materials of the boiler and tube bundle, microstructure of the tube bundle, drum, welding seam, and welding heat affected zone, electrochemical analysis of the tube bundle material at room and high temperatures, as well as fracture morphology and surface composition analysis. The analysis results show that the failure positions are concentrated near the welding area of the tube bundle and the drum, and the cracks expand from the surface of the weld to the inside. Moreover, widmannstatten structure and upper bainite structure appear in the welded and heat-affected zones, which significantly reduces the plasticity and toughness of the materials. The results of electrochemical analysis show that the boiler water has significant corrosiveness to the boiler material at the working temperature of 160 ℃. The morphology of the fracture surface demonstrates severe corrosion. Additionally, stress corrosion cracking occurrs in the welding area due to an intense residual internal stress generated during the welding and the unevenness of the weld surface.