The seepage stability of the overburden layer of a dam foundation is studied in this paper to provide basis for seepage control for the dam. Firstly, according to the geologic and hydrogeologic characteristics of the dam foundation overburden layer, the seepage deformation type is preliminarily estimated based on hydrogeological parameters obtained from site and indoor tests. A 3-D numerical model of the overburden layer is subsequently built to get the characteristics of seepage field and hydraulic gradient based on the above parameters and seepage theory so as to analyze the leakage amount of reservoir water permeating through the overburden layer under different operation conditions. In view of the above study, the simulation results of hydraulic gradient under different conditions are analyzed to assess the seepage stability of the dam foundation, which is further compared with the preliminary estimate so that the final conclusion can be derived.