Abstract: The leakage of reservoir is a principal engineering geological problem of Manghekou Reservoir, Jiyuan City, Henan Province, China. A qualitative study on the probable karst leakage path of Manghekou Reservoir is given. According to the nesting model which has three kinds of scale (watershed, reservoir zone and dam region) and 3D numerical simulation technology, 3D numerical groundwater seepage flow model is built in dam region. The change of seepage flow field, the capacity of leakage and the effect of anti seepage are predicted after the reservoir completed. These provide scientific basis for the design and construction of watertight curtain.