Taking an air-floating rectangular box for example，we deduce the nonlinear boundary condition of air-water interface at the structure bottom considering air compressibility through adiabatic equation.Based on the 2-D potential flow theory using the analytical solution of eigenfunction expansion and progressive matching method，we investigate the amplitude frequency of wave forces with different water depth-width ratios and different drafts.It is shown that the horizontal wave force increases with draft increasing，but the vertical wave force and the moment declines with the increase of the draft;Wave forces decrease with the increase of the water depth，and the change of wave moment caused by vertical wave forces is more obvious than that caused by horizontal wave forces.