Abstract: The deep overburden dam foundation of strong and weak layers is always met in Southwest and Northwest China. Weak aquifer in deep overburden dam foundation has a great influence on seepage. The influence law needs to be explored in depth. Weak aquifer is set at different positions underneath dam foundation in this test. The depth of vertical cuttoff wall is variable. The result of this test shows that when weak aquifer at the middle of dam foundation, cuttoff wall can form semi-closed combined seepage prevention system, which can effectively decrease seepage discharge of dam foundation and exit gradient. When the semi-closed combined seepage prevention system is adopted in Shangjiang dam, seepage discharge and exit gradient meet allowable requirements. If weak aquife can be utilized, the depth of cuttoff wall can be greatly reduced. This study also finds that when the position of cuttoff wall is deeper, the seepage control of semi-closed combined seepage prevention system can effectively reduce seepage discharge and exit gradient. The results can provide some theoretical reference and suggestions for similar projects, so as to reduce the project cost.