Abstract: The interaction principle for equilibrium analysis of multiphase media is illustrated in this paper.Then a uniform differential momentum equation,which governs the pore fluid flow in multiphase media, e.g.either saturated or unsaturated soils,is deduced based on the principle.By assuming that pore water flowing is in laminar flow,i.e.,interaction between fluid phase and solid phase is proportional to water velocity,the Darcy's Law formula can be inferred.In this way,physical meaning of the Darcy's Law for saturated soil is explained.The formula expressing the relationship between the permeability and soil potential and water content in unsaturated soil is also derived.Mme permeability testing results of clay and siliceous powder in steady hydraulic gradient are presented in this paper.These results are well agreeable to the theoretical prediction by the momentum equation.