Increased precipitation and nitrogen (N) deposition have considerable impacts on soil microbial communities in terrestrial ecosystems, ultimately influencing the carbon (C) cycling. However, the specific effects of the interactions between increased precipitation and N deposition on C:N:P stoichiometry in soil microbial biomass remain unclear. A long-term (9 yr) manipulated experiment was conducted to investigate the responses of soil microbial biomass C:N:P stoichiometry to increased precipitation and N deposition of the Desert shrubland in Gurbantunggut, northwest China. The results demonstrated that increased precipitation elevated C, N, and C:N ratios of microbial biomass by 26.9%, 18.7%, and 6.8% relative to ambient precipitation, respectively. In comparison to ambient N deposition, N addition resulted in a 37.5% increase in soil microbial biomass N and a 6.8% increase in the N:P ratio. Particularly, the interactive effects between increased precipitation and N deposition on microbial biomass P and N:P ratio were significant. Moreover, the influences of increased precipitation and N addition on microbial biomass C:N:P stoichiometry primarily occurred through changes in dissolved organic carbon content and extracellular enzyme activities. The findings underscore the crucial role of increased precipitation as a stronger driving force for variations in microbial biomass C:N:P stoichiometry in arid ecosystems, compared to N deposition.