Desertification has become a global social environmental problem and is closely related to fragile ecoenvironment. From the viewpoint of landscape, mountain system lies in the agriculture forest cross zone. The fragility of its ecosystem shows on many hands, such as inner fragility, dependence on vegetation, transition of landscape, backward productive force, etc. Desertification is relatively severe. In this paper, by analyzing the hillside cascade system and the inner feedback mechanism of the system, the operating mechanism of mountain desertification is discussed and furthermore some thoughts about the prevention and control of desertification are raised.