Abstract: Specimens with different welding defects are prepared by simulating boiler pipe-seat fillet weld characteristics. Some Specimens with different welding defects simulated by boiler pipe-seat fillet weld were designed and manufactured. Influences of different welding defects on welded joint of 12CrMoV steel are investigated by loading bending fatigue test, and the fatigue fractures of different specimens are observed by scanning electric microscope. It is concluded that compared to the fatigue lives of the whole-penetrated specimens, those of the whole-not-penetrated, half-not-penetrated, whole-penetrated undercut 1 mm specimens drop by 38% to 46%, but that of the whole-penetrated undercut 0.5 mm specimen increases by above 35%. The results show that the first three welding defects have no obvious differences on the fatigue lives of the specimens, and that the defect of the whole-penetrated undercut 0.5 mm increases the fatigue life of specimen due to its notch enhancement effect. All the specimens fracture near the weld toes, and remarkable tear ledges, fatigue striations and secondary cracks can be discovered in the fatigue crack propagation area. The expanding cracking path doesn’t go through the welding defects of the not-penetrated, indicating that the defect forms have no influence on the fatigue fracture positions. This research provides technical support for the optimization of the welding technology and the safety of the boiler.