Abstract: To address the risk of leakage and durability reduction caused by sealing degradation during the long-term service of shield tunnels, this study investigates the durability of ethylene propylene diene monomer (EPDM) gaskets under water-rich compressed conditions. Water–thermal accelerated aging tests and mechanical tests were conducted under different compression levels to examine the evolution of compression set and compressive performance. A time-dependent aging model was then established using the compression set ratio as the degradation index to relate laboratory aging to the service life of compressed gaskets. The results show that compressive loading markedly accelerates gasket aging and induces substantial irreversible deformation, with the maximum difference in compression set exceeding fivefold. Residual performance decreases with aging time and is jointly affected by aging temperature and compression level. A rapid nonlinear decline is observed during the first 10 years of service, followed by a gradual slowing after 20 years. At the end of a 100-year service life, the residual performance under compression levels of 6, 12, and 18 mm is 0.824, 0.613, and 0.454, respectively. Overall, EPDM gaskets under water-rich compressed conditions exhibit pronounced time-dependent degradation and high sensitivity to compression level, and the results provide a basis for sealing design, service-life prediction, and leakage risk assessment in shield tunnels.