Mikania micrantha, a perennial fast-growing climber and creeper, has widely invaded secondary forests and natural ecosystems in southern China. The understanding of seasonal effects on soil microbial community would help to clarify the invasive mechanism. The present study examined the relationships between soil characteristics, microbial community structure (as indexed by phospholipid fatty acid (PLFA) profiles) and function (as indexed by enzymatic activities) where M. micrantha was invading a native forest community at Shawan-Hill in Shenzhen, South China. Samples were collected four times within one year to study seasonal trends. The results indicated that soils undergoing M. micrantha invasion had significantly different physico-chemical properties, PLFAs and enzymatic activities than soils not yet invaded with M. micrantha. Significant fluctuations in soil microbial community composition and enzymatic activity occurred between seasons. Four enzymes, bacteria, and the ratios of mono/sat and 18:1É9t/18:1É9c exhibited the similar trends of higher value in warm months, whereas the abundance of fungi was higher in cold months. Differences in the structural variables were significantly correlated to differences in the functional variables, as determined by Pearson’s correlation analysis. However, Mikania invasion was found to have more pronounced effects than season on soil physico-chemical characteristics and soil microorganisms. Mikania appears to increase N and P availability, alter microbial community structure and function, which might be the key factors for successful invasion.