Abstract: Spillway tunnel has been adopted in a great number of large hydro-power stations to discharge flood water. For a ski-jump energy dissipator that is often used on spillway tunnels, its energy dissipation significantly depends on the shape of flip bucket. This paper presents a comparative study on different outlet shapes of spillway tunnel, focusing on an analysis of their energy dissipation effects for an actual hydropower station in design. We have conducted hydraulic model tests and suggested optimal shapes of flip bucket through comparison of the impact pressure and flow velocity in the plunge pool. The results are useful for enhancing the energy dissipation effect and operation safety of spillway tunnels.