A demonstration plot in Chongming Dongtan for controlling exotic plant species Spartina alterniflora by using an integrated treatment technique of tussock cutting plus water level controlling was selected to make a 2-year monitoring on the reinvasion process of this plant species. A large number of S. alterniflora seedlings could proliferate and settle down in the hydrological restoring plots by tidal water in spring, and, after two years, the reinvasion community was established, with no obvious differences in its density as compared to the communities in neighboring areas. However, the distance of extension via vegetative propagation was limited. At the plots maintaining waterlogging, the distance of 2-year lateral extension was less than 1 m; at physically isolated plots, there was no any reinvasion of S. alterniflora. The rapid spreading of the seedlings in spring was the key of the fast reinvasion of S. alterniflora, and thus, to completely eradicate the spreading source from neighboring areas, to build isolation barrier to stop the spreading of this plant, and to adopt biological substitute by planting a native species such as Phragmites australis would be the main countermeasures to effectively prevent the reinvasion of S. alterniflora.