A new attempt is proposed to improve the load-carrying capacity of high strength steel undermatching butt welded joints by shape design. Based on the principle that reasonable geometric parameters designs of welded joints can regulate the distribution of stress and strain, reduce the stress concentration factors in low strength weld zone, the geometric parameters of undermatching butt joints, such as weld toe radius, height of excess weld metal and width of the cover passes are designed. A series of tests were completed to investigate the effect of weld shape design on the mechanical properties and security of undermatching butt joints. The results showed that the critical crack sizes of the 0.6~1.0 undermatching joints is greater than or equal to the one of base metal, the joints strengths approach to the tensile strength of base mental, and the fatigue strength is far higher than that of the as-welded equalmatching butt joints.