<p>This study investigates the multi-crack interaction mechanism and merging criterion for an egg-shaped pressure shell fabricated by welding and hydroforming a six-segment frustum. First, an egg-shaped pressure shell welded from six frustum segments was designed, and its residual stress distribution under 1000 m seawater external pressure was numerically simulated. The stress intensity distribution along the weld was determined through joint ABAQUS-FRANC3D simulation. Subsequently, an equivalent specimen replicating the maximum weld stress state was designed, and a finite element model incorporating multiple cracks was established. The stress intensity factors at the crack fronts were solved to quantify the interaction effect of an interference crack on a reference crack. Finally, a novel crack merging criterion was proposed and validated against experimental results, demonstrating strong agreement. This study provides an effective methodology for evaluating multi-crack coalescence in components subjected to fatigue loading.</p>