Abstract: The commonly used empirical formula for permeability coefficient calculation fails to consider the influence of pore structure and mineral composition of clay on the permeability of soil, so the calculated value has a large deviation from the measured value.Therefore, it is necessary to correct the commonly used empirical formula of clay permeability coefficient.Firstly, considering the isolated pores and surface pores of soil as well as the influence of water layer on the permeability of clay, we define the connected pores with the greatest contribution to clay seepage as effective pores, and the calculation method of effective porosity for clay seepage is given.Then the permeability coefficient formula is derived combined with pore distribution characteristics.Comparing the permeability test of undisturbed soil and remolded soil in Nansha District of Guangzhou City, it is found that the deviation between the calculated results by the modified permeability coefficient formula and the measured values was small, which was much better than that by the traditional Stokes pore flow permeability coefficient formula.