Forest vegetation and soil microorganisms are important components of forest ecosystems, and their interaction plays an important role in maintaining the function and stability of these ecosystems. The relationship between diversities of plants and soil microbes has been studied extensively in natural grasslands and forest ecosystems. However, the effect of tree species diversity on soil microbial community composition and its mechanism in the planted forest ecosystem with multiple tree species remains unclear. Therefore, this study focused on the mixed planted forest ecosystem through close-to-nature restoration in south subtropical China, and examined the effect of tree species diversity (1-10 species) on the diversity of soil bacteria and fungi and the main influencing factors, using the high-throughput Illumina sequencing method. The results showed that the α-diversity of the soil fungi increased significantly with the increase in tree species diversity; however, α-diversity of the soil bacteria did not change significantly with the increase in tree species diversity. The community composition of soil bacteria and fungi differed significantly among the different tree species diversity levels. The Pearson's correlation analysis showed that the soil bacterial α-diversity was mainly affected by soil pH and NH₄⁺-N, whereas soil pH and available phosphorus were the main factors affecting the soil fungal α-diversity. Distance-based redundancy analyses showed that soil pH, NO₃^--N, and aromatic C were the main environmental factors affecting the soil bacterial community composition, whereas soil organic carbon, NO₃^--N, fine root biomass, and O-alkyl C were the main factors affecting the soil fungal community composition. Our study indicated that tree species diversity had a significant influence on the soil microbial community by changing the physical and chemical properties of the soil and the root biomass in the mixed planted forest ecosystem with different tree species in this south subtropical region. The study provides a basis for decision-making for increasing ecosystem function through an appropriate assembly of tree species diversity in subtropical planted forests.