[Purposes] Multiple fatigue cracks are prone to initiation at the arc-shaped opening of the diaphragm of an orthotropic steel bridge deck under long-term operation, which poses a great threat to the safety of the bridge. Studying the coupling propagation characteristics of these cracks can better ensure the safety of the bridge. [Methods] The numerical model for the propagation of double fatigue cracks at the arc-shaped opening of a diaphragm was established by combining the theory of linear elastic fracture mechanics and ABAQUS-FRANC3D interactive modeling technology. The crack propagation characteristics of single cracks and double cracks were analyzed to reveal the influence of initial crack shape ratio on the coupling propagation effect. [Findings] The fatigue cracks at the arc-shaped opening of the diaphragm are composite cracks dominated by type I. Compared to the propagation of single fatigue cracks, the coupling propagation of double fatigue cracks has a decreased stress intensity factor, and there is a mutual unloading effect of stress around the propagation area. After a certain number of loading cycles of double fatigue cracks with the same initial shape, the stress intensity factor of one of the cracks first increases and then gradually decreases below the crack propagation threshold. Afterward, the crack propagation form transforms from multi-crack propagation to single-crack propagation. The propagation pattern and results remain the same when the initial shape of the two cracks is changed. The propagation of the main crack is greatly slowed down by the mutual unloading effect of stress during the coupling propagation of the double cracks. The propagation length of the single cracks can be up to 2.19 times that of the double cracks for the same number of cycles. [Conclusions] In this study, the coupling propagation law of multiple fatigue cracks at the arc-shaped opening of the diaphragm is revealed by using ABAQUS-FRANC3D interactive modeling technology with high computational efficiency and accuracy. The research results can provide a reference for the analysis of this kind of fatigue problems.