Our understanding of the patterns and drivers of soil microbial communities at the regional scale remains inadequate although both have been extensively studied in plant communities. In this study, we examined the patterns and drivers of soil microbial communities using data from 24 arid and semi-arid ecosystem sites, covering a broad range of community types, soils, and climates on the Mongolian Plateau. Our findings demonstrated that, at the regional scale, the total soil microbial biomass, fungal biomass, bacterial biomass, and actinomycete biomass increased with mean annual precipitation, soil organic carbon (SOC), total soil nitrogen (TSN), C:N ratio, annual aboveground net primary productivity (ANPP), and root biomass. However, these values decreased with increasing soil pH and mean annual temperature and showed hump-shaped relationships with plant species richness (SR) and ANPP. Variations in soil microbial communities were associated with precipitation, plant community (SR, ANPP, and BB), and soil properties (SOC, TSN, and pH). At the local scale in a typical steppe, water addition by 30 % increased the total microbial biomass by 20 %, bacterial biomass by 16 %, and actinomycete biomass by 54 %. The increased microbial biomass, however, was still 25–41 % lower than that in the meadow steppe that received similar amount rainfall, indicating that, apart from the dominant effect of precipitation, plant community and soil properties could also regulate soil microbial communities. Our findings have important implications for understanding the impacts of climate change (e.g., precipitation) on soil microbial communities and linkages to ecosystem functioning in the Inner Mongolia grasslands and elsewhere.