Abstract: The drainage capacity is an important index in the filtration design of gravel materials. At present, the drainage relationship based on the Terzaghi filtration criterion only considers the gradation relationship between the protective and protected materials, but ignores the influences of the drainage capacity on the filtration protection of gravel dam materials. The anti-permeability failure capability of gravel materials with and without filter layer protection is studied through the indoor permeability, permeability deformation and filtration protection tests. The results show that since the equivalent substitution method does not change the content of the fine materials less than 5 mm in the gradation of the gravel materials before and after the scale reduction, the permeability test results after the scale reduction can approximately reflect the drainage characteristics of the original gradation of the gravel materials. The capability of gravel to resist seepage failure is inversely proportional to the permeability coefficient, and the permeability coefficient is inversely proportional to the relative density. With change of the relative density, it is possible to improve or deteriorate the drainage capacity of the protective materials for the protected materials. In addition to the gradation characteristics, the influences of the relative density on the permeability should also be considered in the design of filter drainage of dam gravel materials to reasonably determine the drainage capacity of the protective and protected materials. The stronger the drainage capacity, the stronger the filter effect of the protective materials on the protected ones. However, the drainage capacity can meet the demand of the drainage performance of the filter design in a certain range, and the high drainage has limited effect on the improvement of the filter capacity. The semi-logarithmic curve can be used to roughly reflect the influences of the drainage capacity on the filtration relationship under the same filterability.