Abstract: In order to improve the bending load-carrying capacity (BLCC) for under-matched butt joints in the elastic stage,the influence of joint geometric parameters on bending stress concentration factor (BSCF) for under-matched butt joint with X-type groove and double side symmetrical type reinforcement was studied based on the finite element method from the viewpoint of materials mechanics. The influence rules of joint geometric parameters on BSCF at the geometric mutations area near fusion line and in the weld bottom center show that the reinforcement plays the most important role in BSCF,the cover pass width has relatively great effect on BSCF and the weld toe radius has minimal effect on BSCF. The influence rules of joint geometric parameters on BSCF at the weld toe show that the cover pass width has some influence on BSCF,the reinforcement and the weld toe radius both have little influence on BSCF. BLCC of under-matched butt joint can be improved by choosing appropriate joint geometric parameters.