Abstract: New energy dissipater of flaring gate pier combined with stepped spillway has effectively solved the problems of high-velocity flow caused by high working head and large unit discharge. This study applied a 3D flow model with RNG turbulence modeling, VOF water vapor two-phase modeling, and PISO algorithm for velocity-pressure coupling on FVM geometry reconstruction of iterative solution, to simulations of aerated flows over a stepped spillway and its energy dissipation behaviors. At the spillway entrance are Y-shape flaring gate piers and their contraction ratios of 0.7, 0.445 and 0.4 were examined and compared. The results show that under different contraction ratios of the piers the cross-sectional average of air concentration fluctuates with a decreasing global trend along the flow, and that the aerated regions over the steps are reduced when the contraction ratio is decreased. Over the initial several steps, the flow is full aerated and the mean air content at a fixed section is higher at a smaller contraction ratio. And the mean content near the top step faces takes an outward transverse trend of first decreasing then increasing from the center-line, while that near the vertical step faces takes a downward trend of first increasing then decreasing. Over the steps near the spillway outlet, the mean air content at a fixed section is lower at a smaller contraction ratio. And the mean content near the top step faces increases outward, while that near the vertical step faces decreases downward. The energy dissipation rate of this stepped spillway is increased with a decreasing contraction ratio of the Y-shape flaring gate piers.