[Background] Arbuscular mycorrhizal fungi (AMF) can promote the nutrient absorption and the growth and development of plants. An invasive plant Wedelia trilobata, invaded in the South China, often distributes in nutrient-poor wasteland, but still grows and expands rapidly. [Objective] To explore whether arbuscular mycorrhizal fungi play an important role in the growth and competition of W. trilobata. [Methods] A greenhouse control experiment was conducted using W. trilobata and its native congener W. chinensis. The effects of AMF (Glomus versiforme), different phosphorus nutrition, monoculture or mix-culture on the growth and the competition abilities of these two plants were investigated. [Results] Both these two plants were infected by AMF, and the colonization percentage of AMF in W. trilobata was significantly higher than that in W. chinensis, especially under low phosphorus. AMF significantly improved the growth and the competition ability of W. trilobata under low phosphorus conditions. [Conclusion] Interactions with AMF not only promote the growth of W. trilobata, but also enhance its competitive advantage. This effect is likely to significantly contribute to the successful invasion of W. trilobata.