Currently, Mikania micrantha (M. micrantha) has invaded Guangdong, Guangxi and other provinces in China, causing serious harm to the forests of southeastern China. Soil microorganisms play an important role in the establishment of M. micrantha invasion, affecting plant productivity, community dynamics, and ecosystem function. However, at present, how M. micrantha invasion affects soil carbon, nitrogen, and phosphorus phase functional genes and the environmental factors that cause gene expression changes remain unclear, especially in subtropical forest ecosystems. This study was conducted in Xiangtoushan National Forest Park in Guangdong Province to compare the changes in soil nutrients and microorganisms after M. micrantha invasion of a forest. The microbial community composition and metabolic function were explored by metagenome sequencing. Our results showed that after M. micrantha invasion, the soil was more suitable for the growth of gram-positive bacteria (Gemmatimonadetes). In addition, the soil microbial community structure and enzyme activity increased significantly after M. micrantha invasion. Correlation analysis and Mantel test results suggested that total phosphorus (TP), nitrate nitrogen (NO₃^–-N), and soil dissolved organic matter (DOM; DOC and DON), were the strong correlates of soil microbial nitrogen functional genes, while soil organic matter (SOM), soil organic carbon (SOC), total nitrogen (TN), and available phosphorus (Soil-AP) were strongly correlated with the expression of soil microbial phosphorus functional gene. Mikania micrantha invasion alters soil nutrients, microbial community composition and metabolic function in subtropical forests, creates a more favorable growth environment, and may form a positive feedback process conducive to M. micrantha invasion.