Abstract: PIC simulation of beam-wave interaction of 8 mm 2-harmonic gyroklystron was carried out. The influences of the high frequency structure, beam parameters and magnetic field on the output power, efficiency and gain were studied. When the coefficient of the magnetic field was 0.511 8, the beam voltage was 70 kV, the beam current was 16 A and the input power was 80 W, the output power was about 430 kW, the transfer efficiency was 38%, the gain was over 37 dB and the 3 dB bandwidth was over 210 MHz.