In the coastal areas, especially the lower Yangtze region of East—South China, the soft soils of the Holocene are widely spread. These soft soils are of littoral facies, floodplain facies, lacustrine facies, and intermountain depression facies, et al. The soft soils in this region, which are of loose structure, high void ratio, and great water content, have poor engineering properties, such as low shear strength, high compressibility and fluidity. These facies soft soil had been taken as examples to study mineral compositions, grain size ranges, physical and mechanical properties of these soft soils and the relations between the indexes of physical and mechanical properties. The results show that engineering behaviors of soft soil are determined by sedimentary facies, mineral composition, grain size ranges and water content.