Abstract: This paper focuses on an intense relativistic annular electron beam in an equi-potential resonant cavity nonlinear self-modulation oscillation. By using the small signal theory, the start oscillation condition is analyzed and the perturbation frequency of the electron start oscillation is given. Based on this mechanism, a new type of high power microwave source is suggested with no additional magnetic field which is mainly made up of a cylinder cavity and a coaxial output cavity. PIC simulation of the nonlinear progress and the effect of input voltage and current on the output microwave power is carried out by using the 2.5-dimensional MAGIC code. The simulation results show that nonlinear instability truly exists in this configuration, the frequency of the self-modulation perturbation dep