In water-rich areas, the deep foundation pit construction often encounters issues of water seepage through the waterproof curtain, posing risks to the stability of the pit structure and damaging the surrounding environment’s safety. Based on a project in Shenyang, the seepage through cutoff waterproof curtains under the geological conditions of water-rich sandy strata is investigated. By conducting seepage model tests, the study delves into the impact of seepage through the cutoff waterproof curtain on the groundwater level outside the pit, aiming to guide the smooth construction of the foundation pit. Based on the principle of similarity, the dimensions of the pit and the waterproof curtain are reduced proportionally to analyze the variation patterns of the groundwater level outside the pit under different seepage areas of the waterproof curtain. Obtainable references for the influence range of the waterproof curtain’s seepage are provided, and pumping tests are conducted to study the water level variation patterns in various observation wells outside the pit caused by seepage at different positions. The results indicate that: As the seepage area of the waterproof curtain increases, the drawdown of the groundwater level outside the pit, the pore water pressure, and the surrounding ground settlement also increase. The ground settlement deformation exhibits a funnel shape under the action of lateral frictional resistance, showing a trend of first increasing and then decreasing with the increase in distance. The influence range of the waterproof curtain’s seepage is approximately six times the thickness of the aquifer. Based on the modified formula for calculating water level drawdown in large-diameter complete wells, a comparative analysis was conducted between the experimental values from model tests and the calculated values. The results indicated that the trends of both were similar, with a discrepancy of 13.7%. This formula is applicable for calculating water level drawdown at different exterior locations of the pit due to seepage through the cutoff waterproof curtain.