[Objective] The effect mechanism of packing particle sizes on the permeability of bioretention cells (BRC) and biofilm patterns was analyzed in order to optimize the particle size and corresponding filling porosity of the BRC packing layer.[Methods] By numerical simulation and experimental measurement methods, fluent software was employed to simulate and analyze the flow field pattern of BRC in the small-scale calculation zone. The streamline diagram, pressure distribution, and change diagram for six different sizes of granule particles (0.5, 1.0, 2.0, 4.0, 6.0, and 8.0 mm) were comprehensively compared. In order to verify the correctness of the simulation results, biofilms with different particle sizes were cultured and operated. The thickness of the biofilm on the particle surface was measured.[Results] When the average particle sizes were 0.5—1.0 mm, the flow field pattern was the best and a smaller eddy current with reasonable structure could be formed. This flow field was the most conducive to water infiltration and material transfer. The measured thickness of the biofilm also showed that when the particle size was 1.0 mm, the growth of the biofilm was the most uniform and the biomass was the largest.[Conclusion] The packing particle size will affect the operation effects of the BRC, and the numerical simulation can provide a reference selection of actual particle sizes in the BRC.