Abstract: Through analysing the coupling coefficient of the generalised telegraphists equations, an optimized 94 GHz gyrotron with gradually tapered cavity is designed. An output power of about 120 kW,corresponding to 41% efficiency, and an oscillation frequency of 94.099 GHz are achieved with a 50 kV, 6 A helical electron beam with a pitch angle of 1.4 at a guiding magnetic field of 3.548 5 T. Compared with the line-joint cavity, the purity of the operating mode is enhanced above 27 dB and the efficiency increases about 7% in the gyrotron with gradually tapered cavity. The efficiency of the beam-wave interaction calculated by the code based on the self-consistent nonlinear theory is in well accord with that simulated by PIC.