Abstract: The essential characteristics of soil permeability are that the seepage channels are made up of soil porosity.Based on the assumption that the particles of coarse grained soils are spherical and the distribution of soil particles is well-proportioned for every fraction,the computing methods for soil particle volume and porosity volume are advised for the same fraction of soils.The soil particle volume and porosity volume of different groups of grains are compared in order from the groups of the minimum grains to those of the maximum grains.If the sum of the particle volume and porosity volume of group is smaller than the porosity volume of the larger grain group,the former is not full of the porosity of the latter.So the estimation method for the maximum grain volume of seepage is proposed in the mean time.Along with the increase of the maximum grain volume of seepage,three states of slurry permeability can be observed:the slurry does not penetrate the samples and the slurry skin is formed;the slurry penetrates the samples and the slurry skin is formed;and the slurry penetrates the samples and the slurry skin is not formed.Finally,it has been validated that a new parameter,pore volume of a single particle under maximum seepage,can reflect the permeability of coarse grained soil.