This paper studied a circular plate's impact force under impact of a mass The formula for the plate's impact force was deduced by using the Galerkin Principle and Laplace Transform. At the same time, the factors that affect the impact force such as the circular plate's radius and thickness, the absorption cushion's stiffness, the altitude of the dropping mass as well as the mass's weight were discussed, and the curves of the impact force versus time were presented for the numerical example. The examples indicated that the above method of calculating the impact force was not only more correct than the conventional contact theory of Hertz, but also more simple and easy to be applied by engineers.