Vegetation degraded significantly in Horqin Sandy Land over the past three decades. Identifying the variability of the key ecohydrological processes (such as evapotranspiration and vegetation growth) that are closely related to desertification is prerequisite for a better control of desertification processes in the future. Based on an ecohydrological cellular automata model, the Water and Vegetation Interactions-based Eco-hydrological Model (WaVEM) was developed and employed to evaluate the effects of the precipitation and potential evapotranspiration changes on the actual evapotranspiration (E_t) and leaf area index,and to analyze the responses of vegetation to the multi-year precipitation reduction. During 1964_2013, E_t and annual maximum LAI (LAI_(max)) had insignificant trends while precipitation changed insignificantly and potential evapotranspiration increased significantly. The precipitation change was the major factor causing the decreases of both E_t and LAI_(max), while the effect of potential evapotranspiration was the second. Multi-year precipitation reduction during 1999-2011 caused significant vegetation degradation in Horqin sandy land. A short-term drought could lead to a sudden decline in vegetation, however, vegetation recovered quickly after the drought.