Abstract: Objective To address the frequent occurrence of water leakage at tunnel portals in operational metro tunnels, reduce maintenance costs and mitigate operational risks, it is necessary to conduct in-depth research on the key inducing factors and treatment technologies for tunnel portal water leakages.Method Based on the demonstration project for shield tunnel portal leakage control in a section on Beijing Subway Capital Airport Express, a comprehensive inspection is conducted on tunnel portal water leakage and other related diseases. The mechanism of recurrent leakage is analyzed, and numerical simulations are employed to further investigate the impact characteristics of cavities behind the lining on repeated water leakage at portals.Result ＆ Conclusion Water leakage at tunnel portals is commonly accompanied by multiple defects such as structural deformation and cracking. The cavity behind the lining is identified as the core factor inducing secondary defects. Structural deformation increases with the expansion of the cavity area, indicating localized failure of internal waterproofing measures. The three-dimensional composite treatment technology-featuring a dual-liquid composite grouting process for cavity filling behind the structure-can effectively form a filling in the voids behind the lining, preserve structural overall stability, establish a 3D waterproof system, thereby effectively resolving the challenges of recurrent water leakage at tunnel portals in operational shield tunnels.