To solve problems in the energy dissipation in a spillway, the hydraulic characteristics of spillway overflow are studied numerically by using VOF (Volume of Fluid) method, cubical mesh and 3D k-ε  turbulence model. The hydraulic characteristics, including flow pattern of jet flow, nappe aeration, pressure intensities on the plunge pool slab, flow velocity in the downstream channel are summarized. Flip bucket of the spillway is optimized by changing the original upper-lower flip bucket to differential flip bucket. Consequently, the flow pattern of nappe and efficiency of energy dissipation are improved, and the pressure intensity on the plunge pool slab and the flow velocity in the downstream channel are reduced. The numerical results of optimized scheme are verified by physical model test results.