Abstract: In order to improve the equal bending load-carrying capacity(EBLCC) of under-matched butt joints in the elastic stage, the shape design of the reinforcement was investigated based on materials mechanics under three-point bending load.The design idea, criterion and realization condition of EBLCC were put forward.Two design methods for the reinforcement shape were proposed after theoretical calculation of under-matched butt joints under three-point bending load.The theoretical analysis results were verified by the finite element method.The research results showed that the reinforcement shape from theoretical calculation was parabolic.The finite element results were basically coincident with the theoretical analysis.The vulnerable area of under-matched welded joints can be transferred from the weld bead to base metal near the weld toe after the shape design of the reinforcement of EBLCC, which improved the bending load-carrying and resistance to bending deformation capacity of under-matched butt joints.The arc shape can replace the parabolic shape for the double-side symmetric reinforcement.The proposed shape design method has potential application in the design of under-matched butt joints of high strength steels.