When the diffusion angle of each side of the wing wall in the gate is 12°, the horizontal distribution of outlet velocity of single-stage stilling basin is easy to be uneven in the medium and low water level period. To solve this problem, based on the physical model test, the law of horizontal distribution of outlet velocity of two-stage stilling basin was studied. The results show that with the decrease of downstream water level, the outlet velocity at the left and right banks of the second stage stilling basin changes in four stages: decrease- increase-decrease-increase, and the initial water level and water level range of each stage were closely related to the Fr. The smaller the incoming flow Fr is, the stronger the horizontal diffusion ability of hydraulic jump in the second stage stilling basin is, and it will be less disturbed by the backflow on both sides of the first stage stilling basin. The range of water level in the second stage will be larger, and it is more suitable for the medium and low water levels. The closer the downstream water level is to the end water level of the second stage, the greater the outlet velocity at both sides of the second stage stilling basin is, and the horizontal distribution of outlet velocity is more uniform. When the incoming flow Fr is the same, the regulating effect of the second stage stilling basin is more significant at low water level, and the energy dissipation rate is greater. The results can be used as a reference for the design of energy dissipation facilities under the gate and the operation management of the gate.