<p>Large-scale reservoir landslides pose significant challenges to traditional management approaches due to their substantial volume and complex deformation mechanisms. Adjusting the groundwater seepage field of reservoir landslides through deep drainage measures is a critical approach to mitigating these large-scale landslides. However, the application of deep drainage tunnel systems in landslide management remains limited, and there is insufficient quantitative evaluation of their drainage capacity and guidance on tunnel layout planning. This study focuses on the Outang landslide in the Three Gorges Reservoir area (TGRA), China, where a deep drainage treatment was implemented. On-site deformation and water-table monitoring data were analyzed, and a finite-element numerical simulation was employed to evaluate the effectiveness of deep drainage tunnels. The results demonstrate that the drainage tunnels effectively control the deformation of the Outang landslide. Furthermore, drainage tunnel 1 (DT1) for landslide mass 1 (LM1) shows superior performance compared to the drainage tunnel 2 (DT2) for landslide mass 2 (LM2). When the reservoir water level (RWL) reaches its maximum, the optimal layout of DT1 is below the slip zone and in the bedrock area adjacent to the RWL. Additionally, deep drainage is more effective in enhancing the stability of landslides with low permeability. These results provide important theoretical support and empirical evidence for the design and implementation of deep drainage systems for large-scale reservoir landslides</p>