Abstract: China has numerous reservoirs equipped with deep outlet facilities, and some ageing gates have received inadequate maintenance over extended periods, making their operational reliability and safety status difficult to evaluate. Under high reservoir levels, inspection, repair, replacement, and testing of these gates are often constrained by the lack of a reliable low-water working space near the gate, while conventional measures such as large-scale reservoir drawdown or prolonged shutdown may seriously affect reservoir operation and comprehensive utilization. To address this problem, this study proposes a novel temporary sealing system comprising an ice plug, the existing lining, and the surrounding rock mass, and investigates its load-transfer mechanism, progressive instability evolution, and design criteria. An analytical anti-sliding equilibrium equation is first derived on the basis of the average shear strength mobilized along the ice–lining interface. The hydraulic thrust acting on the upstream face of the ice plug is balanced by the combined resistance provided by interface cohesion, friction, and lateral confinement from the lining and rock mass, thereby establishing a direct relationship among water pressure, plug diameter, plug length, and interface strength. Physical model tests and three-dimensional numerical simulations are then conducted to verify the feasibility of the proposed system and to reveal its mechanical response under increasing hydraulic pressure. The model tests include a refrigerated-chamber test with an ice plug diameter of 0.5 m and an underwater in situ freezing test with a diameter of 1.5 m. Multi-point measurements of temperature, pressure, and displacement are used to characterize the freezing process, end-face deformation, and sealing response. The test results show that the system can withstand hydraulic pressures on the order of 1.0 MPa under the examined conditions, while test termination is mainly associated with interface leakage or end-sealing failure rather than global structural instability. The numerical model separately represents the mechanical behavior of the ice body, lining, rock mass, and ice–lining interface, and is calibrated against the measured pressure–displacement response. Back-analysis of the first-stage test data gives an equivalent elastic modulus of 800 MPa for the ice, which is adopted in the subsequent simulations. Sensitivity analyses of lining dimensions and mesh size are performed, followed by overload simulations for different plug length-to-diameter ratios and hydraulic pressure levels. The simulations indicate that the temporary sealing system is mainly subjected to axial end-face pressure and radial confinement, and that the ice plug remains predominantly under biaxial compression during loading. The interface shear stress along the plug length generally follows an increase–decrease–increase pattern, reflecting load introduction near the upstream end, stress redistribution in the middle region, and enhanced contact near the downstream end. With increasing hydraulic pressure, local yielding first appears near the upstream end, followed by the gradual extension of a damaged or highly stressed zone, and finally progresses toward a continuous failure path. Sensitivity analysis shows that the length-to-diameter ratio of the ice plug is the key parameter controlling the sealing capacity. Increasing this ratio reduces the concentration of deformation and damage near the upstream end, extends the load-transfer range, and delays the development of overall instability. By combining the analytical criterion, model-test observations, and numerical results, this study recommends a length-to-diameter ratio of 0.4 as the lower design limit for deep outlet facilities under water heads below 100 m in medium- and large-sized reservoirs. In practical applications, a larger value should be adopted when the freezing quality, interface contact conditions, temperature uniformity, construction tolerances, or long-duration stability cannot be strictly guaranteed. This study establishes an integrated research framework encompassing analytical assessment, numerical simulation, and large-scale underwater freezing tests, and provides a quantitative basis for preliminary sizing, safety verification, equipment development, and standardized engineering design of temporary ice-plug sealing systems.