Abstract: Based on the theory of microscopic damage mechanics, a numerical simulation model of crack extension resistance of the girth welds with single edge notched tension and single edge notched bending was established using numerical simulation method. The fracture toughness of girth welds characterized by crack tip opening displacement under different in-plane and out-of-plane constraint condition was calculated based on the conditions corresponding to three types of weld strength:low strength matching, equal strength matching, and high strength matching. Furthermore, a comparison was performed on the rationality of different crack tip constraint parameters for describing the fracture toughness and constraints correlation. The results show that the correlation between constraints and fracture toughness established using the stress triaxiality is weak, while using the crack tip equivalent plastic zone area as the constraint parameter can better describe the effects of in-plane and out-of-plane constraints on the fracture toughness of the material. Therefore, a unified correlation between in-plane and out-of-plane constraints and girth weld fracture toughness was characterized using the area of the zone enclosed by the crack tip equivalent plastic strain contour as the constraint parameter; the robustness of the correlation established between constraints and ductility was further verified using the results of wide plate tensile tests and the finite element calculations of full-size pipe girth welds.