In order to investigate the initiation characteristics and propagation laws of fatigue cracks in the U rib-to-deck weld joints of a steel bridge deck under wheel loads, a segment model of steel bridge deck was established using the finite element method. Deformation characteristics of the structure were analyzed under different wheel load positions, and the correspondence between the wheel load position and typical deformation characteristics was clarified. Local simplified models were then established based on this analysis. Using the local models, the crack initiation characteristics were determined based on stress distribution. Three-dimensional numerical simulation of crack propagation was carried out based on fracture mechanics to obtain the crack propagation law of the U rib-to-deck weld joint under different wheel load positions. The research shows that when the wheel load deviates from the weld, the fatigue crack tends to initiate at the weld root and weld toe of the deck and propagates along the thickness direction of the deck, with the crack extension type being mode I. When the wheel load is located directly above the weld, the fatigue crack prefers to initiate at the weld root of the deck and the weld toe of the U-rib, propagating obliquely perpendicular to the U-rib web. These cracks belong to the I-II-III composite type dominated by Mode I cracks, and their propagation rate is faster.