The seepage calculation of earth rock dam is based on that the anti-seepage body is completely impervious to water in the past. The error of calculated result is inevitable. This paper gave the suspended cutoff wall a reasonable permeability coefficient, and established mathematical model by using finite element method of earth rock dam on infinite deep pervious foundation carried out theoretical calculation.By comparing the two kinds of schemes of suspended cut-off wall,it conducted seepage calculation and analysis in different depths , and draw the following conclusions that the closer the cut-off wall gets to upstream, the better the anti-seepage effect.The effective depth of impervious wall is 6~8 times of water depth before dam.