In order to deeply understand the concept of seepage force in porous media, its mechanical essence is analyzed and its definition is exactly given. From the perspective of micromechanics, the mechanical characteristics of soil particle under percolation condition is analyzed, and the calculation method of singular particle’s seepage force under percolation condition employed in particle flow code in three dimensions(PFC3D) is discussed. Additionally, the analysis model of mesoscopic particle flow for calculating seepage force value is built. By using the fluid-solid coupling analysis function of PFC3D, the features and values of soil seepage force of porous media under percolation condition are studied. Through comparing the value of seepage force obtained by numerical method with that by analytical solution, the results indicate that it is feasible to study the macro-characteristics of driving force in porous media under percolation condition with PFC3D, which is based on mesoscopic theory. Moreover, the seepage force is a synthesis result of normal stress and shear viscous stress acting on the surface of soil particles.