Abstract: The sand gravel has more fine particles and is easy to be compacted. Especially under the action of heavy roller, the pore structure and permeability characteristics of the sand gravel may change after compaction, which will affect the seepage safety of the high concrete face gravel dam. In this research, the permeability test of sand gravel after compaction was carried out to analyze the relationship between the characteristic particle size and the equivalent pore diameter of sand gravel, as well as the influence of different porosities on the permeability of sand gravel, and verify the commonly used empirical calculation formula of permeability coefficient. The research shows that the reduction of the porosity of the sand gravel has a great impact on its permeability characteristics. Due to the large content of fine particles in the sand gravel, and to the fact that the particles are tightly packed after compaction, the permeability coefficient of the sand gravel obtained from the permeability test is 1~2 orders of magnitude smaller than the calculated value of the conventional empirical formula, resulting in a large deviation. As the porosity decreases to a certain extent, the sand gravel shows certain viscous characteristics, the internal void increases, and the permeability coefficient decreases significantly. Based on the formula of China Institute of Water Resources and Hydropower Research, the effective pore reduction coefficient is introduced to modify the formula, and the results calculated from the modified formula are more accurate. This study can provide a basis for the drainage design of high concrete face gravel dams.