Abstract: Multi-source gravity data including shipborne gravity data, modern altimetry satellite gravity data and land measured gravity data as well as the Earth Gravity Model data are integrated into a unified gravity data set for the South Yellow Sea and adjacent areas. The features of Bouguer gravity anomalies of the South Yellow Sea are analyzed. Apparent density inversion based on interpolating cut method and iteration method for big distance downward continuation of potential field is applied to detect the tectonic boundary between Yangtze Block and Sino-Korea Block in the South Yellow Sea. Through the interpretation of apparent density slices at different depth and combined with geological information, we infer that the tectonic boundary between the two blocks in the sea area goes along the fault to the north of the Qianliyan uplift, then extends to the longitude of 125° along NE direction, turns to NNW and back to NE around the latitude of 36° and finally enters Korea Peninsula through the south of Hongsung in NE direction. The Sulu orogenic belt and the Gyeonggi orogenic belt make up the northern margin of the Yangtze Block. Sulu ultra-high pressure metamorphic belt could be compared with the Hongsung complex.