The Bohai Bay is the northernmost region in China where Spartina alterniflora is distributed and diffused. To comprehensively understand the status of S.alterniflora diffusion in the Bohai Bay, based on nearly 20 years of 10-meter resolution multi-source remote sensing data, the random forest classification model, and the Google Earth Engine(GEE) remote sensing cloud platform, the classification and identification of S. alterniflora in the Bohai Bay were conducted using remote sensing techniques. The spatial and temporal characteristics of S. alterniflora diffusion, diffusion patterns, influencing factors, and ecological and environmental responses to diffusion were discussed with the aim of providing data support for the management of S. alterniflora in the Bohai Bay and scientific management of coastal wetlands. The results showed that the annual growth rate of S. alterniflora in the Bohai Bay reached 21.62 hm²/a and the diffusion rate reached 8.85% in the past 20 years, which was still in a rapid diffusion stage in time, and showed a trend of spreading from the Tianjin Haihe River Estuary to the north and south in space. The main areas where S. alterniflora rapidly spread were semi-enclosed tidal flats formed by reclamation, estuarine wetlands, and tidal flats formed by aquaculture withdrawal. The landscape expansion pattern of S. alterniflora has evolved from outlying to edge expansion and gradually to infilling in recent years. The rapid spread of S. alterniflora in the Bohai Bay based on its special advantages of genetic development in the high latitude area, but it was more affected by the large-scale reclamation in the region, inappropriate reclamation projects and other factors on the rapid silting of tidal flat. The spread of S. alterniflora had positive effects on wave prevention and disaster reduction, but also had adverse effects such as crowding birds' habitat. It is necessary to consider the comprehensive effects and urgent needs of S. alterniflora in the Bohai Bay and formulate specific management plans scientifically and reasonably.