Abstract: The overburden layer with large thickness and strong-weak-strong permeability combination is widely distributed in valleys of southwestern China. This paper studies the characteristics and influencing factors of the dam foundation seepage field in the interbed type hydrogeological structure. It uses Visual MODFLOW to simulate the seepage field of 16 working conditions with two types of constant thickness and variable thickness. It analyzes the influence of permeability and thickness of the weak permeable layers to the seepage field morphology, by the change of head difference and hydraulic gradient of roof and floor of the weak permeable layer. The paper reaches the following conclusions. The plane seepage field of dam foundation is strongly influenced by the property of strong permeable cover in upper layer. In constant thickness condition, the head difference of weak permeable roof and floor is related to the permeability ratio of strong permeable layer to weak permeable layer. When the permeability ratio of strong and weak permeable layers is greater than 500, the head difference of weak permeable roof and floor changes slightly. In variable thickness condition, the head difference of weak permeable roof and floor increases with the thickness of weak permeable layer. When the thickness of weak permeable layer is greater than 20m, the head difference changes weakly. Hydraulic gradient decreases with increasing thickness of weak permeable layer. In conclusion, the thickness and permeability of the weak permeable layer play a controlling role in the seepage field in the interlayer hydrogeological structure. The research results can provide support for water prevention and control of dam foundation of water conservancy and hydropower projects in deep covered valley.