Abstract: Karst depressions or confluence areas of gullies provide natural favorable topographic conditions for constructing upper reservoirs in pumped storage projects, reducing excavation volume and construction costs; however, the development of fault fracture zones and karst conduit systems in these areas poses potential seepage risks. Conducting in-depth research on the mechanisms and behavioral patterns of seepage in karst regions for upper reservoirs, identifying main seepage pathways, estimating leakage scales, and formulating targeted prevention strategies are core scientific issues for ensuring project efficiency and economic value. This paper takes a proposed upper reservoir in a karst area as the study object. Based on its geological conditions, karst development laws and rock permeability characteristics, the leakage possibility and leakage pattern are analyzed. Accordingly, FEFLOW modeling is performed for key geological structures such as strata, fault fracture zones, and karst conduit systems. Darcy's Law and Manning's formula are used to simulate the natural groundwater level, which is validated against measured data. The results show that the leakage of the proposed pumped-storage reservoir is mainly caused by pipeline leakage caused by karst pipelines and fissure leakage caused by faults. Under natural conditions, the correlation coefficient R² between simulated and measured groundwater levels in the proposed karst reservoir exceeds 0.75, and the leakage volume at the storage water level of 1155 m elevation is consistent with normative calculations. These results verify the effectiveness of the simulation method for karst seepage analysis in complex geological conditions. Building on this, simulations for the reservoir impoundment scenario are conducted. The Hagen-Poiseuille Law describes the flow in karst conduits (laminar flow), and the Particle Tracking Workflow module traces and labels reservoir water particles to reveal the seepage patterns. Targeted anti-seepage schemes are then proposed and evaluated. The results indicate that without anti-seepage measures, the reservoir is predicted to reach an empty state after 64 days of impoundment at the natural storage level of 1155 m elevation due to seepage. Vertical anti-seepage measures slow the seepage rate but generate new leakage pathways, altering the seepage pattern. Horizontal anti-seepage measures demonstrate excellent performance in both leakage pathways and volume control. The findings provide technical support and references for reservoir construction.