The oscillating hydraulic jump caused by the inflow condition of large single width and low Froude number in energy dissipator for falling-sill bottom-flow has great destructive force to stilling basin of which the size is limited. A joint energy dissipator with a contraction pier in front of the pool has been tested on a hydraulic scale model and proved to have solved the problem effectively. The RNG k-ε model and VOF method were employed to construct two models of stilling basin in the presence and in the absence of contraction pier. The hydraulic characteristics were obtained from the 3D numerical simulation. And the stilling basins in the presence and in the absence of contraction pier were compared in association with hydraulic scale model test data. Results unveiled that insufficient turbulent energy and unstable large-scale eddies are main causes of the oscillating hydraulic jump in the stilling basin with step-down floor. In the presence of contraction pier, two high turbulence intensity areas are formed in the stilling basin. More 3D eddies of smaller scale were formed, enhancing the turbulence intensity and shear strength, disrupting the large-scale eddies of water jump and eliminating the oscillating hydraulic jump effectively.