The waterproof curtain that is used to block groundwater, leaks occasionally during deep excavation and can have a harmful impact on the surroundings. This study adopts a three-dimensional (3D) fluid-solid coupled finite element model (FEM) to analyze the impact due to a leakage in the waterproof curtain during excavation dewatering. In the model, both a user-defined one-dimensional leakage element and a solid leakage element are incorporated to simulate the leakage. The leakage of the curtain in the soil is analysed by monitoring the variation in hydraulic conductivity. The simulated results demonstrate that the leaking rate, leaking volume, groundwater drawdown, and ground settlement are closely related to the hydraulic conductivity of the leakage point. The results also show that the groundwater head outside the pit declines quickly when the leakage occurs, but soil consolidation proceeds slowly. In addition, the characteristics of the soil that is adjacent to the leakage point also have a tangible effect on the groundwater head and ground settlement. The hydraulic conductivity and deformation modulus of the soil has a significant effect on the ground settlement, while the influence of the anisotropy of the soil is relatively low.