This simulation study investigated the function of riparian buffer strips in reducing rainstorm-runoff and purifying pollutants under various conditions of rainstorm types, durations, recurrence periods, slopes of the riparian buffer strip, vegetation coverage, and first flush. Simulation of rainstorm-runoff and buffer strip conditions indicated that the pluviometric runoff coefficient of the riparian buffer strip ranged from 0.29 to 0.55. The riparian buffer strip function in reducing rainstorm-runoff worked better when storm duration was short, and the recurrence period was long. The reduction of rainstorm-runoff was also better when the buffer strips had gentle slopes and good vegetation cover. Pollutant removal rates were:suspended solids 66.41%-90.29%, total nitrogen 44.48%-64.90%, ammonia nitrogen 32.72%-63.68%, total phosphorus 89.83%-95.04%, and chemical oxygen demand 34.32%-66.23%. There is no clear trend of riparian buffer strip's purifying function in different storm types. The riparian buffer strip can purify first flush because of the higher reduction rate to high concentration runoff pollutants. In addition, the riparian buffer strip purifies better when the surface flow velocity is slower.