The majority of earth-rockfill dams in our country are constructed on the heterogeneous infinite deep pervious foundation. According to the selection of different vertical impermeable wall depth in the heterogeneous infinite deep pervious foundation, the paper obtained the conclusion that the trend of marked reduction of seepage of a dam foundation was decreased, when the effective depth of suspended cut-off wall is about 6to 10times water head in front of dam, the amount of seepage in a dam foundation varies less, when the effective depth of suspended cut-off wall is about 18to 20times water head in front of dam, the amount of seepage tends to be stable.