Abstract: The concrete cutoff wall is a primary anti-seepage measure for concrete-face rockfill dams (CFRDs) built on alluvium foundations. As a concealed structure, its mechanical behavior is influenced by both foundation deformation and seepage, which in turn directly affect the effectiveness of seepage control and the overall safety of the dam. This study investigates the mechanical behavior of concrete cutoff walls in CFRDs constructed on alluvial foundations using statistical analysis and numerical simulation. Drawing on monitoring data from multiple engineering cases, the deformation characteristics of the cutoff wall are examined from a statistical perspective. A numerical model incorporating the foundation’s seepage–creep coupling effect is developed. A nine-parameter creep model is employed to represent the rheological behavior of the foundation, and hydromechanical coupling is used to simulate the interaction between seepage and creep. Based on the comparison between field data and simulation results, the stress and deformation characteristics of the cutoff wall are analyzed. The influences of seepage–creep coupling, dam construction speed, construction sequence of the cutoff wall, and wall depth are also discussed. The results indicate: (1) At dam completion, the cutoff wall deforms toward the upstream side; during reservoir impoundment, it bends toward the downstream side. Frictional resistance is the main cause of high vertical stresses. Significant tensile stress appears at the wall–bedrock interface and the base, while high compressive stress is concentrated in the midsection of the wall. (2) External loads amplify the stress within the wall due to the foundation's seepage–creep coupling effect, with creep having a more prominent influence on wall deformation. (3) Rapid dam construction increases both deformation and stress in the cutoff wall during filling. Delaying wall construction reduces stress and deformation at completion but may worsen stress conditions during impoundment. Deeper cutoff walls experience greater stress and deformation, negatively impacting structural safety. Furthermore, fully embedded walls exhibit tensile stress zones at steep slope interfaces and the base, while suspended walls tend to concentrate tensile stress near steep slope boundaries. The findings provide valuable references for the design and construction of concrete cutoff walls in CFRDs on alluvial foundations.