Abstract: The weld joint of large diameter thick wall tube and small diameter tube of boiler always fails in view of fatigue action, and this failure mostly hails from periodic bending stress by small load and long lever arm. In this paper, based on characteristics and service condition of the pipe joint of pressure parts of the boiler, the specimen was designed. By cantilever bend loading mode, displacement-life curve of the fillet weld joint of pipe seat for the boiler, namely D-N curve, were studied by group-method. The results manifested that D-N curve could be used for finite life design of the fillet weld joint of pipe seat for the boiler and D-N data distribution could be expressed by power function rule, which would provide reference for assessment of fatigue life of the fillet weld joint of pipe seat for the boiler.