Abstract: The gradation design of filter materials is one of the key problems in design of earth rockfill dams. The design of the filter materials, besides meeting the standard requirements of soil filtering, water drainage and protecting the protected soil by self-healing after cracking, should adapt to large shear deformation and make the effect of deformation transition. The soil materials are divided into two parts, coarse skeleton materials and fine filling materials. Based on this fuzzy concept, the design idea of the filter materials in the design code for rolled earth-rock fill dams (SDJ218-84) in China is specified as follows: (1) The macroscopic concept of mixed impervious soil containing particles which exceed 5 mm in diameter is discussed. The content η_0.075 for fine particles and the content P₅ for particles exceeding 5 mm in diameter are a pair of eigenvalues and different for different soils. The four classifications of protected soils and their filter relationship in each classification in the filter criterion of US Bureau of Reclamation can still be used, but the particle size D₁₅ should be that of the fine filling materials in filter soil instead; (2) From the classification of leakage deformation in Liujie’s study, the demarcation particle size d _f between the coarse skeleton materials and the fine filling materials, and the critical content η _f for the fine filling materials are proposed. Furthermore, the design basis of filter materials is summarized to be nine conditions; (3) through an example, the practical design method for the filter materials is introduced.