Abstract: The river scouring caused by air jet is important to engineering safety. In this paper, the diffusion law of jet flow in downstream water cushion is deduced based on momentum equation, the influence of water cushion velocity on jet diffusion is analyzed, and the calculation method of bottom velocity under moving water cushion is proposed. It is shown that the moving water cushion increases the jet diffusion distance, promotes the jet diffusion and the jet velocity attenuation, and reduces the bottom velocity. The results of the research are applied to the prediction of the depth of the flushing pit downstream of the spillway tunnel of the Baihetan project. It is shown that the scour depth under the condition of moving water cushion is about 10% less than that under the condition of still water, and the depth of the scour pit formed by the flow on the downstream side of the channel is smaller than that formed by the flow on the upstream side of the river. The method proposed in this paper can well predict the depth of the scour pit at different locations, and the experimental results are in good agreement with the calculated results. The research results can provide a basis for the energy dissipation and anti-scour design of the downstream channel of flood discharge facilities.