Riparian buffer has a key function on rainfall runoff reduction and water purification. SCS-CN model, artificial surface runoff modelling, and water sampling were used to reveal the effects on rainfall runoff reduction and water purification of the riparian buffer of Sanli River. Moreover, influence of the surface characteristics were analyzed. The results were as follows:(1) Rainfall runoff could be reduced significantly and the reduction ratio could be more than 80%. Low antecedent soil moisture, long slope length, and low slope gradient could promote the reduction effect. The reduction effect of bush was larger than that of herb, flower, and pavement. (2) The reduction ratio of vegetation on TP, TN, NH₃-N, COD, and SS could be 85.35%, 13.41%, 68.32%, 87.76%, and 98.5%, respectively. In general, the water purification effect of 60 min was better than that of 30 min. Herb, flower, high grass and bush had large reduction ratio, while gravel had small reduction ratio. The reduction ratio could be promoted by over 50%, when the slope gradient was cut down from 10° to 5°. The addition of slope length showed little influence on water purification effect.