Theoretical research and optimization design of 8mm second harmonic gyroklytron amplifier were reported. A threecavity, second harmonic gyroklystron amplifier prototype was fabricated according to the results of numerical simulation. Initial experiments show a peak output power of 180 kW at 35 GHz with an efficiency of 13% and a gain of21dB. A peak output power of 150 kW with a 3-dB bandwidth of 110 MHz (0.3%) at 58kV beam voltage and 25A beam current was obtained.