Abstract: This study investigated the core area of the Jiangsu Yancheng Wetland National Reserve of Rare Birds based on 12 remote sensing images from 1983 to 2021 as data sources. Specifically, this study explored the spatiotemporal trajectory of Spartina alterniflora expansion and its impact on landscape patterns by combining landscape ecology methods with geographical information system (GIS) technology. The results show that from 1983 to 2021, Spartina alterniflora expanded significantly, leading to a 12.365 fold area increase. It experienced initial expansion, accelerated growth, and stagnation stages, which would be succeeded by the control and elimination stage. Its distribution area manifested a significant linear relationship with time. Its spatiotemporal trajectory was characterized by the migration of the western, eastern, and central clusters. In 1983, 1988, 1992, and 1997, the western clusters migrated primarily toward the southeast. In 2000, 2002, 2006, and 2009, the eastern clusters migrated principally toward the northeast. Both the western and eastern clusters showed a seaward trend. In 2011, 2014, 2017, and 2021, the central clusters displayed a significant landward trend despite disorderly migration. Spartina alterniflora expansion resulted in a cumulative contribution rate of 43.352% to regional landscape structure changes. Its contribution was consistent with its expansion stages, showing a low-high-low pattern. The area of Spartina alterniflora was significantly correlated with the regional landscape pattern index. The landscape pattern of Spartina alterniflora was significantly correlated with the regional landscape pattern, with significant correlations between type-scale indices, including largest patch index (LPI), total edge (TE), edge density (ED), and fractal dimension index of area-weighted mean (FRAC_ AM), and the regional landscape pattern index at the significance level of 0.01. However, the area of Spartina alterniflora was significantly negatively correlated with habitat quality. Overall, the results of this study suggest that the expansion of Spartina alterniflora profoundly affects landscape patterns and functions, warranting control according to local conditions.