Plant invasion initiates with the establishment of an alien species population that begins interacting with the existing community in the invaded habitat. Competitive ability may confer advantage to invasive species during establishment. Autopolyploidy has been shown to significantly contribute to successful invasion of China by Solidago canadensis that is native to North America. But how polyploidization improves competitive ability and determines the dominance of invasive species when competing with a plant community in the introduced range remains unclear. Here, we manipulated the initial plant composition of plowed land and subsequently allowed natural colonization by S. canadensis in a five-year common garden experiment. Diploid, tetraploid and hexaploid populations collected in North America (native range) and East Asia (introduced range) were separately planted and allowed to compete with associated weeds in individual plots. The diversity and compositional variation of the plant communities and the growth characteristics of S. canadensis were investigated in summer and autumn each year. Based on how the community assembled, three outcomes were found: 1) S. canadensis outcompeted local vegetation: tetraploids and hexaploids from the introduced range outcompeted associated weeds and were dominant at equilibrium; 2) S. canadensis coexisted with local vegetation: hexaploids from the native range were competitive but ultimately could not outcompete the local vegetation; and 3) S. canadensis became extinct: diploids from both the native and introduced ranges and tetraploids from the native range went extinct. Concomitantly, diversity was low in the first group and high in the second and third. Therefore, polyploidization contributes to the pre differentiation of competitive ability among native S. canadensis populations, facilitatating the invasion of China by this species. The competitive ability of polyploids was enhanced through possible rapid post introduction evolution after their introduction into China, which could be the crucial factor for successful invasion by S. canadensis.