Abstract: Invasive alien plants pose a serious threat to biodiversity of natural reserves, and understanding the key factors influencing their successful colonization is crucial for their management. Invasive alien plants can lead to soil legacy effects, which impact the germination, growth, and competitive abilities of themselves and other invasive plants. This study selected two invasive plants, Chromolaena odorata and Bidens pilosa, within Encheng National Nature Reserve in Guangxi as research objects. We conducted a comparative experiment using soil from the areas invaded by C. odorata. Four levels of soil invasion by C. odorata, including no invasion (0%), light invasion (<50%), moderate invasion (50%-90%), and heavy invasion (>90%), along with soil sterilization, to explore the effects of soil microorganisms following the domestication of C. odorata on the germination, growth and competition of two invasive alien plants. Our results indicated that: (1) Compared with the unsterilized soil, only C. odorata germinated, and the germination rate, plant height, root length and biomass of B. pilosa were reduced by 63.9%, 68.9%, 58.1% and 95.9%, respectively. (2) The microbial communities in lightly invaded soil showed suppressive effects on B. pilosa, while the microbial communities in the moderately and heavily invaded soil showed promoting effects, indicating that soil legacy effects are responsible for driving the invasive meltdown between C. odorata and B. pilosa. (3) The relative competition intensity (RCI) index indicated that regardless of soil sterilization, B. pilosa exhibited higher competitiveness than C. odorata in the early stages of invasion, with stronger competitiveness in moderately or heavily invaded soil. These findings highlight the crucial role of soil legacy effects mediated by microorganisms in invasive alien plants invasion. Changes in soil microorganisms due to invasive alien plants are responsible for promoting the colonization and invasion of other alien plant species.