Abstract: When a tunnel passes beside a large water conservancy project, the waterproof curtain between them is of? ten designed as a structure with a certain opening, the size of the opening is directly related to its waterproof capacity and its influence on the surrounding groundwater environment. Based on practical engineering, the influence of opening size (buried-depth and density) on dewatering effect and recovery time of the water level is discussed by numerical simulation. The results show that: (1) it can be divided into three zones regarding the impact of opening size on drawdown, i.e. non-affected zone, gradual changing zone and abrupt changing zone; it can be divided into two zones regarding the impact of opening size on recovery of water level, i.e. gradual changing zone and abrupt changing zone; (2) the larger of buried depth of waterproof curtain, the better the dewatering effect but the longer of water level recovery time. In light of dewatering effect, water level recovery time, construction cost and so on, the selection of design parameters of waterproof curtain should be within the gradual changing zone; (3) based on its hydrogeologi? cal conditions, the design parameters of waterproof curtain are proposed, that is to say the depth and density should be within 14-20 m and 0-0.8 respectively, and the validity is verified by engineering app