Impacted by climate changes and human activities, the coastal zone of China has undergone various dramatic changes, among which, shape change of major gulfs (including bays and estuaries) is one of the most important aspects. In this paper, spatial dataset of gulf shoreline and gulf shape in seven phases since the early 1940s was delineated based on topographic maps and remote sensing imageries, and a group of indices including the ratio of natural shoreline, the shoreline utilization degree, shoreline swing direction, gulf area, gulf shape index and gulf centroid were used to analyze the spatial-temporal characteristics of gulfs in detail. It turned out that: in general, shape changes of major gulfs in the mainland of China were significant and complicated during the past 70 years, specifically, visible changes of shoreline structure, especially the decreases in both length and ratio of natural shoreline took place; the shoreline utilization degree increased remarkably because more and more natural shoreline had been artificalized; the fact that shoreline seaward shift caused by sea reclamation occurred in the majority of gulfs; spatial size (area) decreased in most of gulfs due to the fast and extensive sea reclamation; the geometric shape of most gulfs had become more complicated while the discrepancies among all the gulfs were narrowed; the centroid of most gulfs moved toward the sea; gulf- shape variations exhibited remarkable spatio- temporal differences. It is obvious that human activities, especially sea reclamation, have become the key driving force of gulf shape change since 1990. Based on the above- mentioned findings, classification management strategy based on dominant- function oriented gulf classification is strongly recommended, and gulf-based management should be strengthen in order to improve the abilities of Integrated Coastal Zone Management (ICZM) practices in China.