The spatial pattern changes of bays under the influence of reclamation can profoundly reflect how human activities affect the natural environments, which is important to effectively protect and utilize bay resources. Based on 6 Landsat TM/OLI remote sensing images during 1990-2015, this study analyzed the variations of major bays from the coastline and bay surface morphology and explored the correlation between the reclamation intensity and spatial pattern changes for the 12 major bays in the East China Sea (ECS). The main conclusions include that: (1) the length of the main bay coastline in the East China Sea, from 1990 to 2015, increased by 66.65 km. The extensive coastline growth was found during 2005-2010 and the growth reached 38 km. Sansha Bay has the longest coastline (439 km) and the shortest (105 km) was found in Luoyuan Bay; Xinghua Bay experienced the largest coastline growth (54.53 km) in the past decades, and the least was in Luoyuan Bay (25.75 km). In general, the artificial coastline continued to increase and the degree of artificialization had been continuously strengthened. (2) The coastline of the bay continuously moved to the sea, with a distance of 26.93 km (1.08 km/a). The most significant seaward expansions were found in 1995-2000 and 2005-2010, reaching 7.10 km and 6.00 km, respectively. Hangzhou (4.93 km) and Xinghua bays (4.15 km) experienced the largest seaward expansion of coastline, while Xiamen Bay had the shortest (0.55 km). (3) The total area of the major bay waters decreased from 13.85 km2 in 1990 to 12.29 km2 in 2015 in the East China Sea, down by 11.23%. Additionally, the morphological indices of the bays showed a continuous rise trend, which indicates that spatial patterns were transformed to be more complicated. The largest reduction with water area was observed in the Hangzhou Bay (0.726 km2), accounting for 46.69% of the research area. (4) The indexes of artificiality and development intensity showed a continuous rise trend. The utilization degree in the southern part of the study area is higher than that of the northern part, and the interannual fluctuation of the development intensity in the north is much varied. In addition, the bay development is positively correlated with the length of the coastline, the length of the artificial coastline and the shape index of the bay, and negatively correlated with the length of the natural coastline and the area of the waters. As the development intensity increased, the intensity of reclamation activities increased significantly.