Soil and water conservation mechanism of vegetation has been explained through analysing the effects of vegetation at different layers on rainfall. The vegetation cause the result of reduction of runoff amount,runoff energy and runoff carrying sediment. and vegetation improving soil to make a great role of soil and water conservation because of vegetation increasing resistant soil erosion. The research point out soil and water conservation function of vegetation is due to using out runoff amount continually by vegetation at different layers, and the accumulation effects of vegetation at different layers lead quickly reduction of soil erosion amount as to realize bring function of vegetation soil and water conservation into full play.