The study of desert plant community construction mechanisms can help to protect the plant resources of the desert ecosystem and maintain the balance and stability of the system. Based on the combination of phylogeny and functional traits, the plant community of the desertification closure reserve in Bohu County was used as the research object, we set up the samples on different slope aspects (between hills, shaded slope, and sunny slope) of the longitudinal sandy monopolies in the research area, and investigated and measured the functional traits of plant community species and soil factors. Then we examed the phylogenetic signals of functional traits of plant community species, analyzed the expression patterns of phylogenetic and functional structures of plant community species in different slope aspects, and screened the environmental factors influencing species coexistence by using principal component analysis (PCA) and linear regression model. Thus we further revealed the driving factors of species coexistence in arid desert ecosystems in the study area. The results showed that:(1) the vegetation in the study area was mainly dominated by drought-tolerant shrubs and Chenopodiaceae herb; soil factors were heterogeneous in different slope aspects, and soil nutrients and water were more abundant between the hills and shaded slopes. (2) the phylogenetic signal K values of 10 functional traits of plant community species in the study area were all less than 1, indicating that the functional traits were less influenced by phylogeny and species functional traits and did not show phylogenetic conservatism. (3) the phylogenetic structure of different slope aspects tended to diverge, and the limited similarity played a dominant role in plant distribution; the richer soil fertility between the hills and on the shaded slopes was the main reason for the divergence of the functional structure of the species, while the functional structure of species on sunny slopes showed aggregation effect, and habitat filtering was the driving factor. (4) the linear regression model between functional diversity index and environmental factors showed that the effective phosphorus, soil water content, and organic matter were main environmental factors that significantly influenced the species coexistence of plant communities in the desertification closure reserve in Bohu County. This indicates that soil water and nutrients are the main constraints to plant species coexistence in arid desert ecosystems, and the drivers of ecological niche differentiation differ between slope aspects, and plants change their adaptation to environmental changes by shaping their own functional traits phenotypes. Therefore, in the process of sealing and conserving the study area, the soil moisture, nutrients, and soil seed bank of different slope aspects should be taken into account to realize the artificial afforestation and tree nurturing to achieve the improvement of the ecological environment in the area.