Abstract: The leakage of the weld of an elbow in the sequence after pneumatic control valve of drain pipe of high pressure cylinder extraction line occurred in a nuclear power plant. The failure analysis of the weld was carried out by means of the macroscopic morphology and microstructure observation, fracture morphology analysis, and vibration and strain measurement. The results show that the intermittent opening and closing of the pneumatic control valve resulted in the large alternating stresses in the weld, which was greater than the fatigue strength and led to fatigue cracking. The residual tensile stress existing in the weld promoted the extension of the cracks. Turning the intermittent drain into continuous drain by adding one bypass drain pipe on the pneumatic control valve return road could avoid the occurrence of such failure effectively.