Abstract: A crack was found on the weld toe of sensor pipe joint by welding on the LF2 aluminum alloy fuel system pipe after being used for a period of time. The causes of cracking were analyzed by means of macro observation, fracture macro, metallographic examination, welding quality inspection and verification test of welding structure improvement. The results show that the cracking property of the LF2 aluminum alloy fuel pipe joint was fatigue cracking. The crack originated from the stress concentration on the outer surface of the pipe. The stress level at the weld toe of the fuel pipe joint was high, and was superimposed by the vibration stress perpendicular to the axial direction of the pipe, resulted in the weld cracking.