Abstract: In plain river network areas, there is a close connection between groundwater and river channels. When constructing reservoirs in areas with poor geological conditions or where pollution isolation is required, a full-reservoir geomembrane anti-seepage scheme is often used. To ensure the geomembrane's buoyancy stability and prevent air inflation issues, a drainage network is installed under the membrane for drainage and pressure reduction. Relying on a full-reservoir geomembrane anti-seepage plain reservoir in a water diversion project, a groundwater flow simulation method that couples porous media seepage and pipeline flow was adopted to simulate the composite seepage field involving the interaction between the complex drainage network and the reservoir's geotechnical body. By comparing the seepage pressure distribution under different drainage schemes and soil permeability conditions, it is shown that within the closed areas of the orthogonal drainage network, the sub-membrane seepage pressure gradually decreases from the center of the area to the surrounding drainage pipes, indicating significant pressure reduction within the closed areas. The sub-membrane seepage pressure at the dam foot is significantly higher than within the reservoir area, necessitating the addition of drainage pipes at the dam foot. The permeability of the silty fine sand layer greatly affects the sub-membrane seepage pressure, and the sub-membrane seepage pressure should be closely monitored during low water level operations. The study results can provide a reference for the seepage control design and safe operation of full-reservoir geomembrane anti-seepage reservoirs in plain river network areas.