Forest ecosystems are an important part of the terrestrial ecosystem. In forest ecosystems, the plants transmit carbon, nitrogen, phosphorus and other nutrients to the soil in the form of litter. Soil microorganisms, as the main decomposers of litter, affect the energy flow and material circulation of forest ecosystems. This paper took the mixed Quercus acutissima Carruth. and Robiniapseudoacacia L. forest (QR), as well as pure Quercus acutissima Carruth forest (QAC) as research objects. The field fixed-point sampling, indoor analysis, and high-throughput sequencing methods were used to study the characteristics of soil microbial diversity and their effects on the litter decomposition rate. The results were as follows: (1) The litter decomposition rate of QR was higher than that of QAC, the litter decomposition rate of QR was 0.70, and the litter decomposition rate of QAC was 0.62. During the litter decomposition of the two forest stands (QR and QAC), total organic carbon (TOC) and total nitrogen (TN) were released, but total phosphorus (TP) was accumulated first and then released. (2) The two forest stands had the same dominant soil microbial communities, the dominant soil bacterial communities were Actiobacteria, Proteobacteria, Actinobacteria and Verrucomicrobia, and the dominant soil fungi communities were Basidiomycota, Ascomycota and Mortierierellomycota. (3) During the litter decomposition of the two forest stands, the variation range of the soil microbial community richness index and diversity index of QR was smaller than that of QAC. The bacterial community richness index (Chao1 index and ACE index) and community diversity index (Shannon index and Simpson index) of QR and QAC showed a trend of first increasing and then decreasing. The fungal community richness index of QR showed a trend of first decreasing and then increasing, and the fungal community diversity index showed a gradually decreasing trend. The fungal community richness index of QAC showed a gradually decreasing trend, and the fungal community diversity index showed a trend of first decreasing and then increasing. (4) The litter decomposition rate was significantly positively correlated with the soil bacterial community richness index and diversity index, while significantly positively correlated with the soil fungal community richness index. The soil microbial community had an important influence on the litter decomposition rate of QR and QAC. The research results are helpful to understand the relationship between the litter decomposition rate and soil microbial communities in mixed forests, and provide theoretical basis for the research of the influence of soil microbial diversity on litter decomposition in mixed forests.