Using landscape pattern indexes and the methods of centroid position with the help of 3S technology, we examined the landscape characteristics of Spartina alterniflora and its spatial expansion patterns from 2006 to 2015, in the core area of the Yancheng National Nature Reserve (YNNR) in Jiangsu Province. Furthermore, we forecasted its expansion trends based on the GF-2 Satellite Image of 2015. Our results indicated the following:first, the recent landward expansion of the Spartina alterniflora marsh occurred at a faster speed than seaward expansion, its centroid continuously moving landward. Second, over the period from 2006 to 2015, patch density of Spartina alterniflora showed an increase. In addition, the landscape shape became more complex in the tidal flat wetland and the area weighted average shape index, correspondingly increased to 8.63. Third, three expansion patterns of Spartina alterniflora, marginal expansion (ME), external isolated expansion (EIE), and tidal creek-leading expansion (TCLE), were identified through the landscape expansion index and the patch fractal dimension. The patch areas of ME were larger than the external isolated expansion, while the patch number of EIE far exceeded the total number of ME and TCLE. Tidal creeks played a leading role in Spartina alterniflora expanding landward. Finally, based on the three expansion rates measured in our field investigation, the landward spreading trends of Spartina alterniflora marsh was simulated by using buffer analysis in ArcGIS10.2. Simulation results indicated that the area of Suaeda salsa will decrease by 11.45% in the next 5 years. The conclusions of this study will be helpful for understanding the invasive mechanism of Spartina alterniflora in the Suaeda salsa marsh.