Abstract: As one of the effective Low Impact Development measure, the grass swale has been widely used in road construction. In order to analyze the grass swale effects on the surface runoff control under different design parameters, a numerical model coupled hydrology and hydrodynamic processes is used. The reliability was verified against the measured runoff process. The simulation results show that grass swale plays an important role in reducing flood peak and delaying the peak time. The runoff control rates are more significant for grass swale with gentler transverse and vertical slops, and the overflow well located on the upper reaches of the flow inlet. Moreover, when the position of the overflow well varies, the runoff control rate and the flood peak reduction can be changed by 2% to 10% and 13% to 28%, respectively. If the transverse slope changes from 1/3 to 1/7, the runoff-control and flood-peak reduction rates can be increased by 4%-13% and 6%-23%, respectively. Besides, the runoff-control and flood-peak reduction rates are respectively improved by 7% to 16% and 12% to 29%, as the vertical slope changes from 0.5% to 0.3%. The conclusions obtained from numerical simulations provide design guidance of the grass swale and further widen the application.