Abstract: For solving the leakage problom from the tail of the lower reservoir of Ming Tombs Pumping-Storage Station, a cutoff needs to be constructed. Because there is an about 20 m thick alluvial clay layer in the river bed, the cutoff may be inserted into clay layer instead of bed rock, so that two third of the cost for cutoff will be saved. The paper discusses the continuity of the clay layer in the lower reservoir and a formation and distribution model of the clay layer is suggested according to the drilling result, geophysical and hydrogeological conditions. At last, it is pointed out that clay layer is widespread and continuous and it is feasible that cutoff is inserted into the clay layer.