Abstract: Radiation impedance is a key physical quantity,used to describe vibration system.As far as the radiation impedance of stepped circular plate is concerned,the classical theory has difficulty in calculation,due to the fact that the radiation surface is not coplanar,and so the domain of integration is difficult to choose.From the point of view of radiation acoustic power,and based on superposition theory,a superposed method is proposed to calculate the radiation impedance and as an example the radiation impedance of circular plate with one step is obtained.