Abstract: The paper describes the numerical study on the vircator with stepped waveguide. Simulation results demonstrate that the length of stepped waveguide can strongly affect the microwave generation，when the resonance frequency of equivalent column cavity for stepped waveguide is about equal to the frequency of the virtual cathode oscillation, the efficiency of microwave generation is maximum and the spectrum of microwave is more singular, the efficiency of microwave generation varies periodically with the length of stepped waveguide.