Abstract: According to the analysis of 27 microstructure pictures, we found that the cohesive soil in the central South Yellow Sea has three microstructure types: granular link-bond structure, flocculent link-bond structure, and clay matrix structure. The cohesive soil in the study area is mainly of clay matrix structure which has well developed pore and loose structure. The factors that lead to the loose microstructure feature of the cohesive soil have been analyzed for macroscopic aspects (topographic features, sediment source, hydrodynamic environment, deposition rate) and microscopic aspects (grain size distribution, mineral component, chemical composition), the analytical results revealed that flat seabed terrain, weak hydrodynamic conditions, long transportation distance cause this area has small sediment source and low deposition rate, consequently the deposition substances is mainly of silt and clay particles. The content of illite, chlorite, montmorillonite and organic matter is abundant in the cohesive soil, their hydrophilic characteristics make the cohesive soil in this area swell, thus formed the clay matrix structure with well developed pore and poor structure. The results revealed the microscopic mechanism of the macroscopic engineering properties of the cohesive soil in the muddy area of the central South Yellow Sea and deepen the understanding of the engineering properties of marine sediments.