Abstract: In order to make bending load-carrying capacity (BLCC) of under-matched welded joint reach that of base metal, theoretical research on critical condition for under-matched joint to satisfy equal bending load-carrying capacity (EBLCC) condition are conducted in this paper. Minimum reinforcement and minimum weld width of under-matched EBLLC joint have been derived in the elastic stage. Design criteria of under-matched EBLLC joint are proposed in the elastic stage. The feasibility of arc replacing parabolic curve has been analyzed, design method of three-circle tangential EBLLC joint which is suitable for engineering application are given in elastic stage. Experiments verify has been carried out for under-matched EBLCC butt joints with three-circle tangential reinforcement. Experiment results show that BLCC of under-matched joints can reach that of base metal and all bending specimen meet the requirement of the EBLCC design. In this paper design criteria and design method can make under-matched joint can be designed direct in accordance with BLCC of base metal. All of these can provide an important theoretical basis for BLCC design of under-matched welded joint in elastic stage.