Abstract: The material in the weld area of the pipeline is prone to defects. The problem can be solved to a certain extent by applying the over-matched welded joints. The accurate test of fracture toughness of over-matched welded joints is based on the accuracy of J-integral plastic factor. In this paper, the single edge notch tension (SENT) specimen of over-matched welded joints with thickness and width ratio of 1 is researched. The finite element model of SENT specimen without side groove and SENT specimen with side groove depth of 10% are established. The J-integral plastic factor considering crack length width ratio, matching coefficient and weld width is calculated, and the effects of crack length width ratio, matching coefficient, weld width and side groove on J-integral plastic factor are analyzed. The over-matched J-integral plasticity factor equation is fitted. This research provides the basis for the accurate test of J-integral resistance curve of over-matched SENT specimens.