Abstract: In order to improve the fracture resistance of under-matched butt joint,the influence of joint geometric parameters on shape factor of under-matched butt joint with center crack is studied based on the finite element method.The result shows that the reinforcement plays the most important role in shape factor of under-matched butt joint with center crack,the cover pass width has the relatively great effect on shape factor and the weld toe smooth transition radius has little effect on shape factor.The fracture resistance of under-matched butt joint with center crack can be improved by choosing appropriate joint geometric parameters.