A finite element model of the steel plate girder bridge segment was established to study the cracking characteristics of the stiffener-to-web weld. Under the most unfavorable position of vehicle load, the normal stress and shear stress characteristics of web gap under different transverse load positions and the influence of crack length on the out-of-plane deformation of web were analyzed. The stress intensity factors at the crack tip and the deepest point of crack were compared under different crack lengths, which revealed the influence of crack length on the stress characteristics and the growth rate of crack front. The results show that when the crack is along the side of the surrounding weld end under the joint action of bending moment and shear force, the crack is mainly of type I crack, and when the crack is far away from the side of the weld end and extends to the web, the crack depth grows faster. When the crack penetrates the web, the out-of-plane deformation of web increases obviously, the shear stress of the web becomes more obvious, and the crack propagation rate increases obviously.