This study resolves the discrepancies between stability analysis results derived from current dewatering design standards for foundation pits and actual confined-water-induced accidents. By differentiating between local weak points at interfaces and overall pit risk characteristics, we systematically analyze emergency response capabilities under varying stability coefficients against confined water, thereby establishing a four-tier design framework for confined water control in metro foundation pits. For Tier IV criteria (aquifer burial depth ≥25 m and stability coefficient >0.8), we investigate pit scale characteristics and propose a stability criterion incorporating interface side friction resistance. Comparative analysis of historical metro excavation surge cases reveals prediction deviations of -33% and 42% for conventional pressure balance and shear strength methods, respectively. The proposed simplified discriminant reduces deviations to within 8%, significantly improving failure prediction accuracy. Engineering validation under 15-day sustained excavation with a stability coefficient of 0.86 (non-dewatering conditions) demonstrated no safety incidents, yielding a modified stability coefficient of 1.39. This confirms the method's effectiveness and reliability in confined-water stability analysis. Implementation of this framework reduces construction risks, engineering costs, accident probability, and environmental impacts, demonstrating significant benefits across social, economic, and environmental dimensions. The research provides critical theoretical foundations and technical paradigms for analogous geotechnical engineering applications.