In order to analyze the influence of vegetation cover on soil and water loss on the Loess Plateau slope and quantify the threshold of vegetation cover of soil erosion,the mechanism of vegetation cover on soil erosion was analyzed based on simulated rainfall data,and the observation data of field runoff plots in typical areas of the Loess Plateau such as Suide,Xifeng and Tianshui were used.The effectiveness of vegetation cover in controlling runoff and soil erosion under different cover types was discussed,and the lower threshold and upper threshold of vegetation cover in regulating runoff and preventing soil erosion under different cover types were determined.The results showed that:(1) The increases of vegetation coverage increased soil infiltration,reduced runoff,delayed runoff velocity,and increased soil resistance to erosion.(2) Slope runoff decreased with vegetation coverage as a power function or exponential function,and soil erosion decreased with vegetation coverage as an exponential function,power function or a logarithmic function.(3) In general,the lower threshold and upper threshold of soil erosion control by vegetation in different regions were 20%~30% and 50%~70% respectively.(4) Soil texture,vegetation type and even root characteristics of vegetation had important effects on the benefits of vegetation in controlling runoff and soil erosion.The results provide a research basis for soil and water conservation on the Loess Plateau.