Abstract: In high frequency, the high-order waveguide mode is usually employed to increase the size of resonant cavity of gyrotron, but the problem of mode competition could be caused in this case. In this paper, linear and nonlinear multimode theories are employed to analyze the mode competition of the 220 GHz gyrotron oscillator we designed. The starting current and characteristics of beam-wave interaction of the oscillator are calculated, the output power of modes is obtained through numerical simulation. The results show that there are three balanceable states in the gyrotron and the design of 220GHz gyrotron is reasonable.