Abstract: On the analogy of structural elements of plant communities, it is shown that magnitude of community coverage or canopy closure isn't the adequate condition of an antierosive effective community because the community function of soil and water conservation is related to height of coverage layer, high coverage layer is unfavorable to soil and water conservation. Developing degree(coverage or thickness) of coverage layer close to ground is the ultimate essential characteristic and the adequate necessary condition of anti-erosive effective communities. It is also a key to soil and water conservation by vegetation that we promote the development of coverage layer close to ground or pay close attention to changes of it. So it is considered that soil and water conservation function of grass and shrub should be thought highly besides constructing forests in the Loess Plateau.