Due to the aging of the in-service steel bridge deck, the size and density of widely distributed fatigue cracks at the weld gradually increase, leading to the interaction between the near-source multiple cracks. The coupling propagation and combination of multiple fatigue cracks are also promoted. In order to study the coupling propagation effect between multiple fatigue cracks, a numerical analysis model of two collinear fatigue cracks on steel bridge deck plate cover-U rib welding seam is established. Based on the theory of linear elastic fracture mechanics, ABAQUS-FRANC3D technology is used to calculate the stress intensity factor of single crack and two collinear cracks. The influence of crack spacing and size of interference crack on the propagation characteristics of the foundation crack is revealed, and the simulation conclusion is verified by a full-scale segmental test. The analysis results showed that the stress strengthening effect of collinear cracks cannot be ignored, compared with single crack. When all cracks have the same size, it is the most unfavorable case for multi-crack propagation. If the ratio of crack spacing to crack length s/c is less than 0.5, the stress intensity factor and crack growth rate are significantly affected by the coupling effect.