Abstract: The occuring of debris flow in the valley needs three basic conditions at the same time which are rich loose deposits, enough water and slope terrain. In view of the shortage of the gravity flow drainage technology and the need for the rapid drainage of the storm, then it uses the siphon drainage technology of self-dredging ability and water separation. So it can improve efficiency and durability of debris flows control. To choose the diameter of the barrier grid and design the siphon drainage system is the key to ensuring the dredging capability of drainage pool. Using computational fluid dynamics software CFD, a three-dimensional numerical model is established to analyze the hydraulic characteristics of the flow in the drainage pool first. Then mathematical model of sediment incipient motion and suspension is established. It can use the hydraulic characteristics of the flow in the drainage pool to analyze sediment incipient motion and suspension. So the evaluation index about dredging ability of siphon drainage pool is established. Besides, it analyses three essential conditions for sediment to be drained out of the pool with water. It suggests that when installing the siphon for the engineering, the water head difference between inlet and outlet should be above 5m, and the diameter of the barrier grid should be no more than 10mm. This can guide the design of siphon drainage pool in practical debris flow prevention engineering.