Abstract: Orthotropic steel bridge decks have become the predominant structural form for long-span bridges. However, the prevalence of welds in steel bridge decks leads to frequent instances of fatigue cracking at the weld joints due to the high occurrence of welding defects such as initial cracks, inclusions, and porosity on the inner or outer surfaces of the welds. Current research often simplifies welding defects as planar semi-elliptical shapes, overlooking their physical properties, which results in unclear understanding of how these defects affect the mechanism of fatigue crack propagation. To investigate the influence of welding defects on fatigue crack propagation behaviors in steel bridge decks, a numerical simulation was conducted. This study integrated linear elastic fracture mechanics and the FRANC3D-ABAQUS interactive simulation technique. A refined finite element model of a semi-U rib with cracks and inclusions was established using ABAQUS. Inclusions were inserted at the weld toe, and subsequently, this finite element model incorporating inclusions was imported into FRANC3D. An initial fatigue crack was introduced near the inclusion, and variations in the elastic modulus of the inclusion were employed to simulate the effects of soft and hard inclusions. The interaction between fatigue cracks and inclusions at the weld joints of U ribs and deck plates was analyzed. This analysis revealed the impact of welding inclusions on crucial parameters such as stress intensity factors (SIF), crack morphology, and propagation rate. The simulation accurately depicted the dynamic trajectory of crack propagation through inclusions. The results indicated that welding inclusions alter the stress field of fatigue cracks, with stress concentration points located internally within hard inclusions and near the crack tip in the presence of soft inclusions, resulting in I-III type cracks at the weld toe influenced by inclusion defects. The elastic modulus of inclusions was found to significantly influence the characteristics of fatigue crack propagation. Soft inclusions briefly inhibited the stress intensity factor at the crack tip but ultimately accelerated crack propagation rates throughout the crack’s lifecycle, resulting in a 7.8% reduction in fatigue life compared to welds without inclusions. Conversely, hard inclusions extended weld fatigue life by 10.1%. Crack morphology tended to flatten during propagation, influenced by the presence of inclusions and their impact on symmetry. Soft inclusions attracted crack propagation near the inclusion, causing localized concentrated expansion, while hard inclusions induced a deviation in crack propagation direction due to repulsion effects. The effect of hard inclusions on crack shape was particularly pronounced, with a 22% increase in flatness observed at the crack tip near the hard inclusion. Strict control over the distribution and type of inclusions during the welding process of steel bridge decks was recommended. Inclusion effects should be carefully considered when predicting fatigue life in welds.