Abstract: In order to improve the load carrying capacity of mode Ⅰ edge-cracked under-matched butt joint with flat reinforcement and assess its safety accurately, the influences of joint geometric parameters on its shape factor were studied based on the finite element method.The result shows that the reinforcement height plays the most important role in shape factor of mode Ⅰ edge-cracked under-matched butt joint with flat reinforcement, the weld width and weld toe smooth transition radius has little effect on shape factor.It is unreasonable to guide the shape design of mode Ⅰ edge-cracked under-matched butt joint with flat reinforcement by using traditional shape factor without considering the effect of weld reinforcement.The load carrying capacity of mode Ⅰ edge-cracked under-matched butt joint with flat reinforcement can be improved by choosing appropriate joint geometric parameters.