Abstract: The fracture behavior of inclusions in large SA508.Gr3 low alloy steel forgings under welding thermal cycling was studied by simulating tensile test. The results show that the large SA508.Gr3 low alloy steel forgings with circular notches still have good plastic deformation ability under welding thermal cycling conditions, and more than 20% elongation is obtained. Cracks are not easy to occur in high temperature tension. When the micro-metallurgy in the forging base metal has sharp angle and the inclusions exceed the critical size, the base metal will have 0.2% strain loss under thermal cycling, and the cracks are easy to induce at the sharp angle area. In actual production, it is necessary to eliminate the deficiency of sharp angle exceeding critical size in base metal.