Abstract: To enhance the static load-carrying capacity and fatigue resistance of under-matching joints, according to the features of stress distributions in cruciform joint, the structural design thinking and realization conditions of equal load-carrying capacity(ELCC) were proposed, the weld shape proposals to minimize stress concentration for the K groove cruciform joint were analyzed and determined, the relationship equations between the shape parameters of joints and stress concentration factors at weld toe and weld root were built by the FEA calculating and regression analysis. The shape parameters of ELCC under-matching cruciform joint which yield strength mismatch ratio is 0.571 were designed. FE analysis results show that this design significantly delays the increase speed of stress at weld toe and weld root of the ELCC cruciform joints during tensile and bending loading, the plastic deformation of the weld bead does not occur before plastic deformation of base metal. The peak values of equivalent plastic strain firstly locate at weld toe and then transfer to the base metal outside the weld toe, and the fracture mode can change into a much safer general yield fracture. This design is very important to the application of the under-matching cruciform joint of high strength steel.