To elaborate on the river bed scouring in front of the sluice of low-head hydro-projects, physical and mathematical models are built to study the three dimensional flow in front of the sluice. Movable bed physical model is used to observe the development process of the scouring pit. Furthermore, fixed bed physical model and three-dimensional turbulence model are employed to simulate the 3-D flow field at the initial, middle and steady period of the scouring. The main conclusion is obtained as follows: at the beginning of scouring, as the flow is obstructed by the sluice, vortexes are formed by the diving flow and a complicated three dimensional flow is generated, thereby causing a large amount of sediment activating in front of the sluice structures. Accordingly, it results in scouring in front of sluice structures rapidly. During the middle and later period, the diving flow in the scouring pit is gradually weakened along the water depth direction, hence the vortex disappears, and the development of the scouring pit slows down and finally reaches equilibrium.