Abstract: It was observed that there were two types of advancing fronts for Spartina alterniflora seedlings to invade and expand in Dongtan Nature Reserve of Chongming Island in Shanghai, i.e. S. alterniflora + mudflat (S+M front) and S. alterniflora + Scirpus mariqueter + mudflat (S+S+M front). The expansion pattern of S. alterniflora seedlings at the advancing fronts was studied by setting up the permanent sample plots and continuous observation. The study showed that the S. alterniflora seedlings began to expand and settle down at the advancing fronts in May, the density and the expanding distance of seedlings in the ‘S+M’ model were significantly greater than those of in the ‘S+S+M’ model. The survival rate of the seedlings was high after having settled down, and there was no significant difference in the seedling survival rate between the two types of advancing fronts. The seedling survival rate at the ‘S+M’ front was 80.6%-86.7%, while it was at the ‘S+S+M’ front. The S. alterniflora seedlings could quickly expand and form patches by tillering, and the average annual tillering number at the ‘S+M’ front and ‘S+S+M’ front was (30 ± 4)-( 40 ± 5), and (34 ± 6)-( 37 ± 11) respectively. The mean expanding distance of S. alterniflora seedlings at the ‘S+M’ front was （25.4 ± 3.1）m/a after one growing season, while there were only odd patches formed at the ‘S+S+M’ front, whose expanding efficiency was significantly lower than that of the former. The study showed that the ‘S+M’ front was the major area for the quick expansion of S. alterniflora in Dongtan Nature Reserve of Chongming Island, where there formed continuous expansion edge, the native plant species such as Scirpus mariqueter played an important role in constraining the expansion of Spartina alterniflora at the salt marshes in the Yangtze River estuaries.