The degeneration of alfalfa (Medicago sativa L.) caused by soil desiccation can significantly affect the distribution and diversity of the herbage community in the semi-arid Loess Plateau. However, the mechanism by which soil moisture redistribution caused by topographic features at the hillslope scale influences the degradation of alfalfa and the distribution of other herbage has received limited attention. In this study, the Longtan watershed was selected as the study area. The above-ground biomass of alfalfa and diversity of herbage were investigated, and soil moisture content at a depth of 0-200 cm under different topographic conditions was monitored. The relationships between topographic features, above-ground biomass of alfalfa, soil moisture content, and herbage distribution and diversity were quantitatively analyzed by using ANOVA, Pearson's correlation analysis, and CCA. The following results were obtained:(1) Topographical features significantly affected vegetation community characteristics. Significant differences in the above-ground biomass of alfalfa were found between different slope aspects and slope positions. The biomass of alfalfa on the middle-hillslope and foot-hillslope positions was significantly higher than that on the upper-hillslope in west-and east-facing hillslopes. However, the spatial variability of the biomass of other herbage, species richness, and diversity indices differed with the spatial variation characteristics of biomass. (2) The above-ground biomass of alfalfa was significantly positively correlated with soil moisture content at a depth of 80-200 cm, whereas the correlations between biomass and soil moisture content at a depth of 0-20 cm and 20-80 cm were not significant. (3) The topographical features, soil moisture content at different depths, and biomass of alfalfa explained 87.8% of the variation of other herbage in the alfalfa degeneration communities. Furthermore, the total variation of the slope aspect, biomass of alfalfa, soil moisture content at 0-20 cm, and soil moisture content at 20-80 cm explained 79.3% of the variation in plant communities. The topographical factors caused the spatial and temporal variation in soil moisture content, which affected the degree of alfalfa degeneration through the variation in alfalfa's above-ground biomass in the semi-arid loess hilly watershed. The distribution and diversity of herbage were determined by the degree of alfalfa degeneration and the soil moisture content at 0-80 cm under different topographic conditions.