Based on the field survey and the interpretation and analysis of satellite images about the population expansion patterns of Spartina alterniflora and Phragmites australis at Jiuduansha shoals in 1997-2004, and combined with 3S techniques, a cellular automata (CA) model was built to simulate the population dynamics of plants adaptable to salt marsh. The results showed that the model could well simulate the population expansion pattern and trend of S. alterniflora and P. australis at Jiuduansha, and strongly supported the hypothesis of space pre-emption and range expansion with simple advancing wave fronts of these two species. The native species P. australis shared the same niche with the exotic species S. alterniflora, but the range expansion rate of S. alterniflora was 2-4 times faster than that of P. australis. With the accretion of the Jiuduansha shoals, a rapid range expansion of S. alterniflora was predicted to last for a long period in the future. The CA model built in this study could gain valuable insights into the relationships between population expansion pattern and its ecological processes of exotic plant species, and was of significance for wetland biodiversity conservation and resources management.