It is showed by many studies that vegetation has very strong effect on water and soil conservation. When the total vegetation coverage density of plant community is great than 40%,vegetation can diminish more than 50% of the total soil erosion with respect to naked land.If the total coverage density of plant community is great than 80%,vegetation will basically control all the soil erosion.Now,under the condition of human disturbing,the real vegetation coverage density on loess plateau is generally lower than 40%.If there were no human action of clamate and geology,,under the same condition the total vegetation (whether forest or steppe) great than 80% on loess plateau according to the ecologic theory and the coverage density would be remnant good vegetation in some rare areas on loess plateau.Through the comparison of the real vegetation and the possible vegetation under the condition of no human action to all kinds of land,we come to the conclution that the accelerated erosion by human action is about 90% of the total erosion at present on loess plateau.