[Objective] To analyze the regulatory effect of grassed swales on road surface runoff in order to provide reference for managing rainfall flood in Beijing City. [Methods] The grassed swales were constructed, and the volume and quality of water flowing was designed based on the data of the average rainfall situation and water quality of road surface runoff in Beijing City recorded during 2010-2012. [Results] (1) The effect on reducing the volume of surface runoff was stronger under rain events with short durations and low intensities. When the duration of the simulated rain event was set to three hours and the return period of rainfall was set as 0.33 year, the 50 m² grassed swales can reduce 66% of the total volume of surface runoff generated from a 50 m² road section. The intensity of a rain event has stronger influence on reducing the volume of surface runoff than the duration; (2) The grassed swales could improve the water quality but could not remove the pollutants entirely. The removal rates of ammonia nitrogen, total phosphorus, and chemical oxygen demand(COD) was 20%, 35%, and 22%, respectively; (3) The vegetation cover had an impact on the effectiveness of the grassed swales. When the vegetation cover increased by 60%, the volume of infiltrated water increased by 8% for a simulated rain event with 0.33-year return period and one hour of duration. (4) If two-meter wide grassed swales can be built along all roads in the six urban districts, they can reduce the road surface runoff resulted from rain events with one-year and two-year return period in 24 hours by 96.3% and 56%, respectively. However, for extreme precipitation such as rain events with 50-year return period, the reduction of road surface runoff is only 13%. [Conclusion] The grassed swales had certain effects on reducing the peak flow, delaying the time of runoff, and improving the water quality of runoff in Beijing City, but it is not an effective management measure to control the urban road surface runoff water, when used alone.