Radiation impedance is an important parameter for evaluation of acoustic conversion efficiency of a sound and vibration system. The high power flexural vibration circular plate with stepped thickness has impartant applications in fluid, especially in air due to its large radiation area. The classical theory has difficulty in calculating the radiation impedance of the stepped circular plate due to the fact that the radiation surface is not coplanar, so the domain of integration is difficult to choose. From the point of view of radiation acoustic power, and based on superposition principle, a method is proposed to calculate the radiation impedance of a stepped circular plate in this work. As an example, calculation of the radiation impedance of a circular plate with one step is given.