The twin-plate breakwater is a new type of harbor engineering structure using twin horizontal plates to attenuate waves and permeate seawater. The horizontal plate expounded in this paper uses steel box-type plate structure. According to the physical model tests, this paper investigates the influence of different supporting forms, different thicknesses of steel plates, different numbers and model numbers of H-beam stiffeners on the stress and deformation of the horizontal plate by the finite element software ANSYS. Comparing the numerical simulation results with those by the traditional calculation method, it provides a reference for the structure design and engineering applications of twin-plate breakwater.