[Objective] To accurately master the variation law of the filtration flow field, and to provide a theoretical reference for the deeper understanding of the change in the flow in the biofiltration system.[Methods] Based on the particle swarm pressure expansion theory, characteristics of the infiltration were studied mainly in consideration of the fluid-solid coupling effect.[Results] A four stage hypothesis in the percolation process of the biofiltration system was put forward and verified by the experiment.[Conclusion] Either for the process of water seepage or sewage seepage, the permeability coefficients changed from falling to rising and then decreasing, which is caused by the particle swarm pressure expansion.