A through crack was found at the weld toe of the welding joint between boss and pipe after a long distance transportation,the cause of failure was found through the metallographic test,chemical analysis,fracture analysis and other test methods. The results show that there are defects in the structural design,the thickness of the nozzle is thinner,However,the wall thickness of the boss is relatively thick. And the upper branch of the pipe is not well fixed. And also,there is a cyclic load in the welding joint during the transportation.Meanwhile,there is no smooth transition on the surface of the fillet weld and this problem has not been detected subsequently. All these factors lead to stress concentration at the weld toe,and the fatigue crack is initiated under the cyclic load. In addition,the wall thickness of the nozzle is thinner,so that the crack spreads quickly through the pipe wall,causing fatigue fracture.