Abstract: Underwater dumping and filling of loose particles is often used in the construction of water conservancy projects such as cofferdams and dams. The compactness of the backfill directly determines the deformation and stability of the project. Experimental and theoretical research was conducted under the prototype condition of a hydropower station dam project with underwater filling in the Han River. Based on the CKY200 large-scale geotechnical centrifuge model test platform of Changjiang River Scientific Research Institute, a single embankment vertical blockage underwater filling test device in a centrifuge field is developed. Using this experimental device, the distribution laws of the compactness of the dumping body under the factors such as the gradation of loose particles, the depth of dumping and filling and the height of upper loading are studied. When the water depth is 12 meters, the average relative density obtained by underwater filling is about 0.27~0.36, and increases to 0.52~0.68 for an additional 14.3 m overlying load. The particle size distribution of sand and gravel plays an important role in the underwater filling density. When the particle gradation of the filling materials is good, the underwater filling materials have a higher compactness, which can reach a dense state. When the gradation is poor, the compactness is lower. The greater the depth of filling, the greater the compactness formed. When the vertical compression is applied to the upper part of the backfill, the compactness of the backfill, especially in shallow areas, is significantly improved. The research results provide a basis for the design and construction of underwater dumping and filling projects, especially for fully utilizing the local granular materials.