Abstract: Vegetation buffer zone is a powerful ecological measure to control agricultural non-point source (NPS) pollution. Its purification capacity is affected by many factors. The specific conditions of different pollution source areas should be considered in the design and construction of the vegetation buffer zone. Taking the farmland runoff in Qiandao Lake as an example, the VFSMOD model was used to analyze the influence of vegetation buffer band width, slope and rainfall on the reduction of inflow sediment. The reduction and change of the inflow total phosphorus (TP) load were estimated and fitted. The simulation results showed that the reduction capacity of the vegetation buffer zone was positively correlated with the width of buffer zone and negatively correlated with the rainfall and slope; when the slope of the vegetation buffer zone was 3%, the pollutant reduction effect was the best; the reduction of pollutants was generally concentrated on the front 10 m section of the vegetation buffer zone. According to the simulation results, a design work curve based on the width of vegetation buffer zone in the rich, flat and dry years in runoff areas of different lengths was proposed. During the design, the corresponding buffer zone width could be flexibly determined according to the expected reduction target and the actual runoff area site conditions. This study could provide technical guidance for the width design of vegetation buffer zone when it was used to purify farmland NPS pollution.