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The objective was to analyze soil microbial community dynamics and their responses to changes in vegetation and soil properties after Robinia pseudoacacia afforestation along a chronosequence. We investigated changes in vegetation communities, soil properties and soil microbial communities 5, 15, 25, and 35 years (Y) after R. pseudoacacia afforestation on cropland on the Loess Plateau. Soil microbial community compositions were analyzed using 16S rRNA and ITS high-throughput gene sequencing. The diversity and richness of understory vegetation community decreased with restoration stage, and available phosphorus and ammonium contents in soil were consistently low. The bacterial communities converted from Acidobacteria- to Proteobacteria-dominant communities within 25-Y but transitioned again to Acidobacteria-dominant communities at the 35-Y sites. Ascomycota and Zygomycota were the dominant fungal phyla at all sites. Compared to the cropland, fungal community composition changed at the 5-Y sites and the bacterial community composition changed at the 25-Y sites. R. pseudoacacia afforestation significantly altered soil bacteria richness rather than its diversity. The planted R. pseudoacacia rapidly altered the soil fungal community composition and altered bacterial community composition at the 25-Y. The changes in soil bacterial communities were driven by the phyla of Actinobacteria, Gemmatimonadetes and Nitrospirae and lagged behind the changes in vegetation communities. Phosphorus was a principal factor in shaping microbial community composition.