Abstract: The welded joint of 1Cr18Ni9Ti stainless steel pipe used in an aircraft broke during bending fatigue test. By means of macro analysis, micro analysis and metallographic examination, the fracture causes of the welded joint were analyzed. The results show that the unreasonable welded structure (the welded heat affected zone was in the transition position of the cross section) was the main reason for the decrease of fatigue life of the 1Cr18Ni9Ti stainless steel pipe welded joint. There was incanplete penetration in the welding area, so it is necessary to optimize the welding process.