Abstract: The Bohai Sea is a marginal inland sea, enclosed by Shandong Peninsula in the South, Liaoning Peninsula in the North and the Yanshan mountains in the North-West. On the background of global climate warming, the Bohai Sea is believed under a trend of sea level rising or marine transgression. Based on the water depth data obtained from the south of the Bohai Sea by the investigation cruise in 1992, and the water depth data of 2006, and also long term water depth data of 20 observation sections covering the Yellow River delta area, the authors found that over the past 20 years, the water depth of the southern Bohai sea is getting shallower. Considering the comprehensive effects of sediment input, basin subsidence, and sea level rising in the south of Bohai Sea, this paper concluded that the water depth of the south Bohai Sea is gradually decreasing at the rate of between 0.44~3.49 cm/a in recent 20 years and the Bohai Sea is in a process of land advancing.