In the field of water conservancy engineering, especially in projects such as large river interception, flood rescue, and guide dike construction, understanding the underwater packing behavior of building blocks under the action of water flow is of significant importance. This study aims to investigate the effects of different building block shapes (sphere, cube, pyramid, and cylinder), friction coefficients between blocks, and water flow velocities on underwater packing behavior using the coupled PFC-CFD method. The results show that the packing body formed by interlocking pyramidal blocks exhibits the highest packing efficiency and density. As the friction coefficient between blocks increases, the packing density decreases. With increasing water flow velocity, the height of the packing body gradually decreases until the structure collapses to a more stable height. These findings provide theoretical references for the design and construction of underwater engineering projects.