The hydrodynamic behavior of large turbid current in stratified reservoir is depicted through numerical calculation results, and the succeeding long-term turbidity is quantitatively analyzed. The large volume current with dense sediment flows towards the downstream along the bottom of reservoir, and the backwater after clashing dam contributes much to the mixing of turbid water with surroundings, causing prolonged turbidity in the reservoir. This mechanism implied possible treatments to reduce the volume of the turbid water. Numerical experiments indicate that successfully purging the turbid water by a low layer outlet, such as a selective withdrawal device, reduces the residual turbid water in reservoir by 88%. Further calculations demonstrate that changing the stratification condition or the outlet elevation in reservoir is conducive to controlling the behavior of sediment flow, hence alleviating the turbidity problem after flood.