Abstract: The excavation chamber opening in underwater shield tunneling is a critical challenge in risk management during tunnel construction, and maintaining the stability of the excavation face at shutdown work site is crucial for ensuring the safe excavation. This study proposes an adaptive lower bound finite element analysis method that considers the effects of seepage, establishing a stability analysis model for the excavation face during pressurized chamber operations in underwater shield tunneling. The model analyzes both active failure mode and passive failure mode of the excavation face under seepage influence and determines the corresponding ultimate support pressure. The results indicate that seepage force significantly alters the failure modes of the excavation face and greatly increases the risk of instability. Based on the undersea shield tunnel project, a dynamic setting method for support pressure during pressurized chamber operations is proposed, utilizing lower bound finite element analysis. Compared to traditional conservative methods, this approach safely and effectively reduces safety redundancy by over 48%, and the optimized support pressure is only 50.7% of the traditional value. In addition, the overall excavation chamber opening operational time is shortened to 1 to 3 days. The proposed method provides guidance and a basis for evaluating the stability of the excavation face and optimizing support pressure settings for excavation chamber opening operations.