This paper reviews the development and discusses the fundamental theory of particle method. According to different Cu value’s grain size curves, three numerical models have been settled by using implicit solver embedded software-PFC which is based on DEM for fluid-structure interaction calculation. In order to get the criteria of seepage failure in granule system, the real-time force chain and flow vector under different hydraulic gradient have been recorded. Process shows that particles move when hydraulic gradient rises but the granule system’s failure would not happen unless an enough high hydraulic gradient value is reached. The result points out that difference between unstable granule system under seepage and the stable granule system under seepage, whose force-chain and flow vector is stable and process convergence can also be got after several time-steps calculation is made, can be the proof whether the granule system fails or not.