Abstract: Under the moving water condition with low water flow of 1.74 cm/s, three planting densities of Hydrilla verticillata were set, including 0.9(ρ1) and 1.9(ρ2) and 3.0/dm ² (ρ3), respectively. The purification effect of different planting densities of Hydrilla verticillata for urban river water was simulated. The results showed that compared with ρ1 experimental group and ρ3 experimental group, the ρ2 experimental group had the superiority in purification of NH₃-N, COD_Cr and TP, with the average removal rate of 87.79%, 67.43% and 82.48%. According to the Environmental Quality Standards for Surface Water(GB 3838-2002), after the experiment, the original surface water was purified from Classification Ⅴ to Classification Ⅲ. It was demonstrated that the Hydrilla verticillata with planting density of 1.9/dm ² had better purification effect, under the low water flow condition.