Abstract: The effects of nitrogen fertilizer (0,15,30,50,100,150,200,300 kg N/hm²) and water on soil microbial functional diversity in Stipa baicalensis steppe were explored for 2 years by the Biolog method. Compared with non-irrigation,water enhanced soil microbial metabolic activity and the utilization ability of carbon sources. The interaction of irrigation and the N₁₅ level of fertilizer significantly increased the utilization ability of carbon source and functional diversity of soil microorganisms. Higher N fertilizer (N₁₀₀,N₁₅₀,N₂₀₀ and N₃₀₀) significantly reduced the richness and evenness index of microorganisms,but increased the dominance index. Irrigation and N fertilizer led to changes not only of functional diversity,but also of carbon source utilization mode and metabolic function of the microbial communities. Principal component analysis (PCA) showed that there was a very significant difference in the carbon source utilization mode of the microbial communities between the lower fertilizer treatments and higher fertilizer treatments indicating that N fertilizer was the main impact factor. Carbohydrates,amino acids,intermediate metabolites,and secondary metabolites were the main carbon source utilized by the microbial communities. In addition,organic carbon,total N,and available N in soil had a close relationship to microbial functional diversity.