[Purposes] The permeability model of coarse-grained soil is relatively mature. However, it is difficult to accurately predict the permeability coefficient of clay, and applying the improved classic Kozeny-Carman (KC) permeability equation to clay has practical significance. [Methods] A method is proposed to define the pores between agglomerates and within agglomerates based on pore size distribution curves of different pore ratios in the literature. Under the framework of the dual pore model, the effective pore ratio and effective specific surface area are calculated by deducting the pores within the aggregates that do not affect seepage. Finally, the classic KC permeability equation is modified. [Findings] Three kinds of clay in the literature are selected to verify the accuracy of the improved equation. The experimental values, predicted values from the classic KC equation, and predicted values from the improved KC equation are compared. It is found that the prediction effect of the improved KC equation is greatly improved, which is consistent with the experimental values. [Conclusions] The improved KC permeability equation based on the dual pore model can be applied to predict the permeability characteristics of clay within a certain range, and it has certain practicality.