Abstract: Seepage deformation or seepage failure is a special bad geological processes formed in result of the intense development of suffusion.The paper combine the weak intercalation existed in the dam foundation of a large scale hydroelectric project,and study on the mode and degree of seepage deformation of this weak intercalation by field sampling test.Based on the result of field test and the arrangement of the hydraulic structure and the head difference between the up reaches and the lower reaches,the paper simulate the hydraulic gradient distribution in week intercalation and in super and base rock mass of the weak intercalation adopted the method of three-dimensional finite element numerical modeling.The result of test and calculation is provided as the base of seepage control design of dam foundation.