Based on coastline, water depth, and remote sensing image data of Sanmen Bay over the past 50 years, this research analyzed quantitatively coastline temporal-spatial evolution characteristics and underwater topography evolution characteristics. The results showed that the total coastline length of Sanmen Bay first decreased and then increased during the last 50 years. The most obvious change of total coastline length occurred from the 1960s to the 1990s. However, in this period, the natural coastline had a higher percentage and a slower change rate. After the 1990s, with artificial interference gradually increasing, the coastline experienced a change process of the curving coastline becoming a straight coastline and moving outward continually. Although the length variation is small, the spatial form of the bay has changed significantly. After 2007 the artificial coastline accounted for more than 70%. From the 1960s to 2015, coastline and 0 meter isobath showed the trend of moving outward. The bay area decreased and most channels deposited. In general, the underwater topography in 2015 was more complex than that in the 1960s. Human activity had a critical role of affecting branch development in the Sanmen Bay area.