Through physical model experiment and considering the characteristic of the main discharge area and return flow zone, the authors mainly analysed the influence of flow characteristic on the river bed and the reason of scouring on downstream river bed of Nanjindu Reservoir. In this experiment, the NDV was used to measure three dimensional flow velocities in the right region. The results show that the characteristics of flow velocity distribution is the main reason that causes scouring in the return flow area of river bed while turbulent energy is the main reason that causes scouring in main discharge area of river-bed.