【Objective】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。【Method】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. Three local simplified models were then established accordingly. Using the local models, the crack initiation characteristics were determined based on stress distribution and three-dimensional numerical simulation of crack propagation was carried out based on fracture mechanics.【Result】The results show that under the deck dominated deformation condition, the maximum principal stress at the weld toe and root of the deck is significantly greater than that at the weld toe of the U-rib, and strength factor Ⅰ is much higher than Ⅱ and Ⅲ after crack generation; Under the condition of U-rib dominated deformation, the maximum principal stress at the weld root of the deck and the weld toe of the U-rib is perpendicular to the thickness direc-tion of the U-rib, and Ⅱ and Ⅲ account for 20% ~30% of Ⅰ.【Conclusion】When the wheel load in the real bridge 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, and their propagation rate is faster.