<p>In water-rich sandy strata, water and soil inrush in shield tunnel may occur, causing structural damage and even collapse. A comprehensive understanding of the mechanism behind soil erosion and tunnel deformation during the disaster was required for guiding disaster-relief in practice. This study investigated the disaster evolution mechanisms based on a water and soil inrush accident at Tianjin Metro Line Z1. To address limitations in current methods for simulating large deformations in full-scale models, an S-CEL method (coupled Eulerian–Lagrangian method integrated with seepage analysis) was proposed. The S-CEL method successfully reproduced the entire accident process, enabling systematic analysis of the generation of new leakage points, soil loss patterns, and structural deformation mechanisms. It was found that disaster escalation was primarily driven by progressive generation of new leakage points. These leakage points developed longitudinally with tunnel deformation, accelerating both soil loss and tunnel damage. Sustained sand loss induced by progressive leakage generated plastic failure zones in surrounding soils and erosion voids at clay–sand interfaces. The presence of erosion voids and plastic failure zone may lead to the ineffectiveness of both dewatering and grouting. Temporary reinforcements in shield tunnel can effectively control structural deformation, thereby delaying the generation of new leakage points. This created crucial time windows for emergency sealing operations.</p>