Abstract: The grain size composition and thickness of filter layer and its structure have significant influence on the seepage characteristics of dam. For a clay core dam project, the particle gradation of filter material on both sides of the clay core (upper and lower cladding lines) was designed, and the particle gradation of the material was prepared. Four groups of seepage deformation tests were carried out, and the critical hydraulic gradient, permeability coefficient and seepage failure types of the test materials were obtained. The double-layer filter structure was designed according to the test results. Eleven combinations of the thickness of the two-layer filter material (the thickness vary from 0.5 m to 2.5 m), the seepage characteristics of the dam were analyzed by finite element method. The results show as follows: (1) The grain size of the first filter is fine, there is little influence on the permeability coefficient of the particle size in the inclusion line of soil material, the influence on critical hydraulic gradient and failure gradient is large, and the form of seepage failure is changed from soil flow to piping; the second filter sand gravel particles are coarse, and the content of fine particles below 5 mm is very small, and the permeability characteristic is determined by the content of coarse grain material, which is transitional failure. (2) When the total thickness of the filters is invariant (3 m), the change of the thickness combination and particle size of the two filters has little effect on the seepage per unit width and the height of the overflow point of the core. The first filter thickness increases from 0.5 m to 2.5 m, the core and the filter material exit gradient shows a nonlinear growth. The exit gradient of the first filter is increased by 92.7%, and second filter by 70% respectively. The particle size change of the first filter is less than that of the second filter. (3) The thickness of the first filter is suggested to be 1.0~1.5 m, the corresponding thickness of the second filter is 1.5~2.0 m; and the particle size selection of both layers of filter material is close to the lower envelope line, and the coarse particle size is the best.