Abstract: To study the invasion mechanism of Conyza sumatrensis,which now dominates the flanks of the tributaries near the Three Gorges Dam. By bioassay, the allelopathy of water extract from C. Sumatrensis that grew along the roadside, in the open wasteland and at forest edge was investigated. The results showed that: (1) In the germination experiment of cabbage (Brassica chinensis) ,the water extract of C. Sumatrensis mainly showed inhibitory effect on the cabbage seed germination and seedling growth. The inhibitory effect enhanced as the concentration increased. Higher concentration of the water extract of C. Sumatrensis could inhibit the cabbage seed germination and seedling growth. (2)The differences among the cabbage seed germination and seedling growth which are treated with the water extracts of aerial part of C. Sumatrensis from different habitats were greater than that treated with the water extracts of underground part of C. Sumatrensis from different habitats, indicating that the allelopathy of water extract from C. Sumatrensis aerial part was greater than its underground part. (3)The allelopathy of water extract of C. Sumatrensis was different among the three habitats ,viz. Roadside > open wasteland > forest edge. (4)Correlation analysis showed that there was a significant positive correlation between the allelopathy of C. Sumatrensis and the relative abundance of native species at the three habitats. The abundance and cover of native species decreased with the increase in the abundance and cover of C. Sumatrensis. These results suggested that differences in invasiveness among habitats were due to the differences in allelopathy at the different habitats. (5)The allelopathic potential of C. Sumatrensis does exsit and it could be one of the important reasons of its successful invasion.