Abstract: The coastline of Sanmen Bay in Zhejiang Province, East China was classified as rocky coastline, sandy coastline, muddy coastline, artificial coastline, and estuary coastline by human-computer interaction based on the Landsat satellite images of 1985, 1995, 2005, 2013, and 2020. The landuse types in the coastal zone of Sanmen Bay were classified as water bodies, forest, bare land, urban, and agriculture using interactive supervision. The results represent that rocky coastline and artificial coastline in the bay have been changed significantly in the past 40 years. Most prominently, the artificial coastline was increased by 80.3 km while the rocky coastline decreased by 79.3 km. The urban and water bodies were the major landuse of recently reclaimed area, which has extended the coastal zone of Sanmen Bay by 130 km² in accumulation in the last 40 years. The conversions from bare land to urban, bare land to agricultural, and forest to urban are the primary land transfer types in the Sanmen Bay coastal zone. In the past 40 years, the fractal dimension of Sanmen Bay coastline has been decreased from 1.22 to 1.201, and the proportion of artificial coastline and urban has been increased by 13% and 22.57% respectively, in general trends of seaward advancement and rising artificiality/urbanization. Human activities are primarily responsible for the changes in the coastline and coastal zone land use of Sanmen Bay. With the implementation of protection and rational use of the coastal zone, the development and exploitation of the Sanmen Bay coastal zone has been gradually improving and stabilizing.