The fatigue parameters of welded joints made of FV520B were obtained considering the real operating situations. The fatigue fracture types were divided into two groups. The scanning electron microscope (SEM) was applied to carefully study the fatigue fracture surfaces so as to investigate the fatigue failure mechanisms. The microstructures of the base metal and the weld seam were studied by the metallographic microscope (MM), SEM and TEM in support of the dissimilar mechanical property analysis. The present paper concluded that the fatigue microcracks mainly initiated from the surface near the weld toe and inclusions in the weld seam, the heat-affected zone (HAZ) disappeared due to the postweld heat-treatment processes, and that the fatigue resistance of the base metal was better than the weld seam.