The anti-seepage effect of the cement-soil mixing pile impervious wall was studied, providing further optimization indicators for the design of impervious wall. The finite element model of the seepage flow was established and verified by the measured data. Different scenarios were simulated for the construction position and penetration depth of the cement-soil mixing pile impervious wall in the earth dam. The results shows that after the impervious wall is added, the seepage stability increases and the seepage control effect is obvious. The closer the cutoff wall is to the upstream of the dam body, the better the seepage stability of the dam body; the deeper the penetration depth of the cutoff wall, the better the seepage stability of the dam body. It is reasonable to place the cutoff wall in the middle of the dam body near the upstream, but considering the construction requirements in the actual project, the position of the cutoff wall should be properly moved back at the axis of the dam, which has little influence on the seepage flow.