Rhizosphere microbes are critical for plant health and ecological functioning in terrestrial ecosystems; however, our understanding of microbial co-occurrences and assembly processes in the broomcorn millet rhizosphere remains limited, especially in distinct regions of northern China. In this study, we examined the network topologies and relative contributions of assembly processes in the broomcorn millet fields of northern China. The environmental factors, diversity and taxonomic and functional compositions differed significantly among the northeast (NE), north (Nor), and northwest (NW) regions. Mantel tests showed that the bacterial and fungal community diversities in the Nor and NW regions were mainly associated with soil nutrients (i.e., available phosphorous, NO3, NH4, and organic matter). Additionally, the topological properties of the bacterial and fungal networks varied with regional differences, revealing that these networks in the Nor and NW regions were more complex, and that bacterial and fungal species within the networks co-occurred more frequently in the region. Moreover, we confirmed that the average relative abundance of operational taxonomic units (OTUs) in the bacterial and fungal networks in the NE and Nor regions occupied a higher proportion in the module hubs, whereas more OTUs in the connectors were enriched in these networks in the Nor region. Modules comprised OTUs belonging to Proteobacteria, Actinobacteria, Chloroflexi, Acidobacteria, and Ascomycota among the three regions. Generally, stochastic processes are relatively more important in constraining assembly across the three regions; however, dispersal limitation governed critical turnover in the bacterial community assembly in the NE and NW regions rather than in the Nor region. These results contribute to the understanding of the processes mediating microbial community assembly and the distribution pattern of the microbial community across the three regions.