Abstract: In accordance with the fracture toughness test standard of BS7448, J-R curve tests were conducted with multiple specimen with welded joints of D406A ultra-high strength steel. According to the standard, the specimen with prefabricated fatigue crack has a standard 3 points bending shape that has a rectangular section of B×2B(B:specimen thickness)and notch orientation of thickness direction. The load and load-line displacement of weld and heat affected zone were tested, and the values of J integral were calculated. Finally, J-R curves and the critical value of J were obtained through the data points best fitted. Thus the problem of failure to test the critical stress intensity factor in welded joints directly by lack of thickness of specimen was coped with and the reliable evidence was offered for the design of solid propellant rocket engine.