Abstract: Aiming to study the influence of rain garden design parameter on the hydrology effect of stormwater retention, we analyzed the long-term effect of a rain garden operation using the DRAINMOD model based on an experimental study, the factors considered include the rainfall characteristics, rain garden storage depth, runoff catchment area ratio and other design parameters. The results show that, DRAINMOD can simulate the hydrologic processes of rain garden reasonably well; the continuous simulation using the long-term meteorological data (1951-2007) showed that the average volume reduction of stormwater runoff was 18.5%; the rain garden treated runoff volume accounted for 76.1% of the annual total. There was a critical value for the rain garden storage depth, above the value, increasing the rain garden storage depth had no effect on rain garden retention capacity. Increasing the runoff catchment area ratio increased both the rain garden treatment volume and the overflow volume. Adding a 30 cm internal water storage (IWS) in the rain garden decreased the subsurface drainage by 19.2%; and increased the volume reduction to 33.5%. Rain garden with IWS may promote the volume reduction and water quality improvement, producing positive effect on urban stormwater management and non-point source pollution control.