Abstract: In view of the early failure of the fillet weld cracking in the lug plate of the third-stage superheater tube of the boiler, in this paper, macroscopical examination, metallographic analysis, energy spectrum analysis, hardness test, tensile test, impact test were used to analyze. The results show that slag inclusion and hot crack are produced in the fillet weld of the third-stage superheater lug plate during the welding process, the local stress concentration provides the internal stress of crack propagation, and the component segregation of the welding joint causes the decrease of plastic toughness, under the interaction of long-term complex thermal stress and mechanical stress, the crack continues to expand to the weld surface, resulting in the early failure of the fillet weld of the third-stage superheater lug plate.