In order to reveal mechanism of nutrient transfer ,two plant (alfalfa and green foxtail), four plant density (0, 80, 120,160 plants square metre), two treatment（complete plantlet and underground biomass）was carried out by simulated rainfall experiment. The results showed that: grass could control erosion effectively, which the effect of plant to reduce erosion was related to the underground biomass closely and the effect of aboveground biomass was not significant. Nutrient Loss with erosion of bare slope was 1.49～5.90 times (total nitrogen), 1.24～8.61 times (total phosphorus), 1.17～1.82 times (available potassium) than vegetated slopes, however, concentration difference of nutrient loss with erosion between bare slope and vegetated slopes was not obvious. Grass controlled nutrient loss with erosion by reducing the amount of sediment mainly, which underground part of foxtail was more effective than alfalfa and aboveground biomass of alfalfa was more effective than foxtail. Meanwhile the role of underground part to reduce nutrient loss was gradually significant with the plant density increased. Grass could reduce the nutrient loss with runoff effectively, and the total nitrogen loss ratio of slope with underground biomass and complete plants was 1.02～1.05 (green foxtail), 1.10～1.14 (alfalfa), indicated grass control nutrient losses with runoff mainly by underground parts, which underground biomass of foxtail was more effective than alfalfa. With the plant density increase, effect on controlling total nitrogen and total phosphorus loss with runoff by underground biomass gradually strengthened but the available potassium didn’t have significant change. The dynamic change of runoff nutrient concentration decreased, than remain stable, which the power function best fit. The change process of total nitrogen and total phosphorus were related to plant situation closely, and total phosphorus changed with plant was not obvious.