Abstract: The U-boundary in the South China Sea is the boundary of China's rights. It is a key area for safeguarding national maritime rights and interests and promoting the strategy of building a maritime power. Understanding the underlying geological structure characteristics of the U-shaped maritime border line in the South China Sea is of great scientific significance. This paper focuses on the geological structure characteristics of the U-boundary in the South China Sea, and obtains the depth of the U-boundary (belt) by using global bathymetric, gravity and magnetic data. Combined with the different nature of the boundary around the South China Sea, the structural characteristics of each section of the U-boundary are analyzed, and the relationship with the rupture and evolution of the South China Sea is analyzed. Through research, we believe that the water depth of the U- boundary (belt) in the South China Sea presents the characteristics of "shallow in the west, flat in the south, deep in the east", ranging from 0 to 5000 meters. The gravity anomaly is negative as a whole (-100 mgal-0 mGal), and there are spot-like positive anomalies affected by seamount. The positive and negative anomaly characteristics of each segment are positively correlated with the terrain. The positive magnetic anomalies in the U-boundary line (belt) of the South China Sea are positive as a whole, ranging from -50 to 100nT, reflecting the magnetic changes and spatial occurrence of the rocks below the seabed. The consistency of U-boundary line strike with submarine geomorphic tectonic units and the section characteristics of each section are also analyzed. In the west section, the strike is consistent with the Honghe fault system and the west margin of the South China Sea. The U1 and U2 sections have compressive folds and negative flower-like structures under the background of strike-slip extension. The southern U-boundary overlaps with the slope break of the southern shelf of the South China Sea and the Nansha Trough, and shows a large-scale rupture unconformity in the section of the early-Middle Sino-World plane, and the strike of the U5 section is consistent with that of the Manila Trench, and the South China Sea plate subducts under the Philippine Sea plate on the section. The U-boundary is located on the gradient zone of abnormally intense gravitational changes at the western, southeastern and eastern boundaries of the South China Sea. It represents the strong undulations of topography and landforms and serves as the boundary of structure and the natural dividing line. The data basis and geological data reference of U-boundary line are enriched to provide data support for maritime delimitation.

Key words: U-boundary of SCS, Marine geological structure, Gravity and magnetic anomaly, 5-zones distinction method of U- boundary