Abstract: Focusing on the issue of water leakage in subway tunnels in coastal areas, this study investigates the leakage conditions during both the construction and operation stages of multiple lines of the Fuzhou Metro. The causes of water leakage are analyzed, and prevention measures are proposed. The study results indicate that during the construction stage, water leakage in subway stations is more severe than in shield tunnels, whereas during the operation stage, the opposite is true. The main leakage areas in subway stations are sidewalls and baseboards, while leakage in shield tunnels is concentrated at segment joints. The primary causes of station leakage are construction-related, including poor pouring quality of enclosure structures, damage to waterproof membranes, and improper treatment of construction joints. The causes of shield tunnel leakage can be categorized into four aspects: installation and construction, transportation and maintenance, material quality, and structural issues, with installation and construction problems being the most critical. To mitigate water leakage in shield tunnels, the structural design should account for construction proficiency. When construction levels are low, flat joints should replace shear key joints to significantly reduce segment damage and water leakage.