In order to explain the gradient of plant communities and plant species diversity, we sampled vegetation in the Alex Left Banner, inner-Mongolia. Canonical correspondence analysis(CCA) was applied for gradient analysis and environmental interpretation, and geostatistical analysis for predicting maps, along with analyses of growth form composition, species richness, and indices of α and β diversity. The results were: (1) plant species α and β diversity fluctuate sharply. (2) The richness of herbaceous plants increased and shrubs decreased along the longitude gradient, whereas, along the latitudinal gradient, the richness of herbaceous plant species decreased and that of shrubs increased. However, there was an exceptional zone with a relatively low richness of herbaceous plants and shrubs between 38°N and 39.2°N. (3) α diversity positively corresponded with longitude but negatively with latitude. β diversity showed that the similarity ratio of species components among different plots decreased along the longitude gradient and replacement ratio increased. Community composition gradually simplified with the latitude increasing. (4) The C0/ (C0+C1) values of Shannon-Wiener index and Simpson index were all between 0.25 and 0.75 which indicated that α diversity spatial correlation was middling. The CCA analysis showed that geographical factors significantly affect species diversity; the effect of longitude was stronger than that of altitude. The research of the dynamics of plant species diversity along the gradient and spatial distribution in the Alex Left Banner is crucial for understanding and conserving biodiversity of this region.