Abstract: When the operating frequency gets very high, the machining error may make that the resonant frequency of the cavity in gyrotron oscillator deviates the design value, or can not meet the engineering requirement. The approximate formulas considering the effects of the symmetrical metal-ring in the resonant cavity of gyrotron oscillator are derived. The resonant frequency dependence for the gyrotron oscillator is obtained and discussed. The cold measecrements show that the theoretical results are reasonable.