Abstract: In order to study the loss of root stones under the action of water flow in dam buttress, based on the Flow-3D software, a mathematical model for studying the loss characteristics of root stones is established by using the GMO model and the RNG k-ε model. The numerical flume test of water flow acting on the cubic block is carried out. The water forces acting on cubic block simulated by the numerical model and calculated by the formula are basically the same, which indicates that the numerical model is reasonable. Then this numerical simulation method of root stones lost is appplied to an actual dam buttress project. The result shows that the most serious area of root stones lost around the dam buttress is the front head of the dam, and followed by the upper cross corner area. It can be found that the movement of root stones in the front head of the dam is the earliest, the rising rate of water drag force is the fastest and the distance of movement is the farthest. The movement of root stones in the upper cross corner area near the front head of the dam is early and the loss of root stones is serious, while the movement of root stones in upper cross corner area near the upstream face area is later and these stones move to downstream gradually as the water flow velocity increases. Root stones in the upstream face area and down cross corner area are less affected by water flow, with basically no root stones lost. The root stones lost don't continue to move with the water flow, and most of them scatter in the dam crotch.