The air cushioning action during the horizontal impact of a flat plate on a water surface is investigated both theoretically and experimentally. In the presented theory, flow of the air layer between the flat plate and the water surface is analyzed by means of the characteristic curve method for the one-dimensional compressible fluid, while the water surface is determined with the aid of the boundary element method by disregarding the gravitational effect and the compressibility. In the calculation of air flow, the initial values and the boundary condition at the plate edge are investigated by introducing acceptable assumptions. The calculated time histories of impulsive pressure are in good agreement with experimental results.