Abstract: Based on features of stress distributions in butt joint, the guide line of equal load-carrying ability about the weld and the base metal was proposed, i.e., to make the stress concentration factor at weld root equal to the mis-match ratio (ratio of yield strength of weld and base metal)by designing the geometric parameters of joint.The empirical equation for the relationship between geo-metric parameters of joints and stress concentration factors at weld root was also built by FEA calculating and its regression analysis.In order to apply such guide line to welded structure of high strength steel, the geometric parameters in undermatching butt joint were designed under this principle about the weld and the base metal in equal load-carrying ability.FEA results show that this design not only made mis-match ratio of undermatching butt joint be selected in large range without reducing the applied stress, but also change the fracture mode of joint from the bad partial yield fracture at the weld zone to the good entire yield fracture of joint.This design is very important in the application of the undermatching butt joint of high strength steel.