In the underwater riprap project，block stones with different particle sizes are often used to form block stones for throwing. In the process of underwater throwing of block stones with multiple particle sizes，the block stones will drift due to the interaction between water flow and the block stones，so it is difficult to accurately locate the falling point of the block stones.On the above problems，this paper uses the finite volume method and the discrete element method based on Hertz-Mindlin contact to study the CFD-DEM coupling of the drift process of the multi particle size block stone group after throwing，and obtains the horizontal displacement，settlement time，diffusion process and impact point distribution of the multi particle size block stone group under different incoming flow conditions. The results show that in the same flow field，because the drag force component of the flow field is relatively small，the drift distance and settlement time of coarse-grained block stones are less than those of fine-grained block stones. The block stone groups with different particle sizes are irregularly distributed at the bottom of the riverbed after throwing，and the drift distance generally increases with the increase of velocity.