Abstract: Unfavorable geological conditions such as deep overburden layers and large hidden faults in the Qaidam basin severely affect the seepage stability of the foundation and storage function of a dam. This study examines the cutoff wall measure for reducing seepage flow in a deep overburden layer with hidden faults, and uses a finite element method to analyze the seepage characteristics of this anti-seepage measure and its seepage control effect. Results show that adopting a suspended cutoff wall of 70 m deep, the Wayan reservoir meets the needs of seepage stability and water storage. If the seepage coefficient of a fault is large, the cutoff wall needs to pass through the fault and go deep into the dense rock mass, and its depth into the fault should be determined by combining the seepage flow rate and rock mass compactness. Also, we can decrease the permeability and seepage flow through grouting. For either of these two measures, the cutoff wall should avoid being extended into an interface of two materials widely different in permeability to prevent local erosion under the large hydraulic gradient around its tip, and its safe and stable operation must be ensured.