The choice of energy dissipater types is crucial to the design of water conservancy projects. Here, the characteristics and variation patterns of water flow in the slot-type flip bucket in the discharge process are studied using RNG k-ε turbulence model and volume of fluid (VOF) method for the simulation of the three-dimensional water gas two-phase flow in the spillway and plunge pool. The results show that the water depth, flow pattern and flow angle of the numerical simulation results are in good agreement with those of the physical model test, which verifies the feasibility of the numerical simulation. On this basis, the shape and position of the water tongue in the air, the flow velocity in the contraction section of the slot, the pressure distribution law and the characteristics of the outgoing flow field are analyzed, with which the position of the maximum pressure at the slot and the variation law of the water tongue velocity in the air are determined. The results indicate that the narrowing effect of the slot-type flip bucket is obvious, the positions of the maximum water tongue velocity in the air are basically the same, and the velocity difference at each point at the same vertical position decreases along the way. The velocity at the bottom plate of the slot section of the spillway shows a parabolic distribution law, whereas the pressure shows a distribution law of three intervals and three trends. The research results can provide a reference for the design of energy dissipaters of the same type in practical engineering projects.