Abstract: Automatic welding technology is widely used in the field welding of large-diameter and high-grade steel pipelines in China. Welding defects such as misalignment and cracks caused by various factors have seriously affected the deformation and bearing capacity of pipeline welding joints. In order to clarify the change law of the welding joint of X80 pipeline with root welding crack, the strain capacity of the girth welding joint of pipeline is simulated numerically. Here, the welding joint of a single-V grooved pipeline with an outer diameter of 1 219 mm was studied. It was assumed that a maximum 3 mm misalignment allowed in engineering was located at the joint and an undetectable surface crack defect, 25 mm × 2.5 mm at maximum, was at the fusion line of root welding. Meanwhile, the differences between the constitutive relations of materials in the four areas of root weld, heat affected zone, base metal and filler weld, were considered. Base on that, the effects of loading mode, yield-strength ratio of base metal and softening rate of the heat affected zone on the strain capacity of welding joints were analyzed quantitatively through numerical simulation. The results show that the change law of strain in welding joint of X80 pipeline with root welding crack under various service conditions can be calculated through the established finite element model. Moreover, the critical strength matching ratio satisfying the engineering application requirements is given with the critical toughness value of 0.25 mm and the strain capacity requirement of 0.5%. Generally, the research results could provide reference for the safety assessment and engineering application of strain capacity of welding joint of X80 pipeline with root welding crack.