Performance properties of a frequency doubled harmonic gyro-twt amplifier is studied in this paper. Numerical simulation show that using a 130kV, 20A electron beam with a velocity ratio of 0.8 and an axial velocity spread of 1%, a saturated bandwidth of 10% can be obtained in an uniform waveguide, the peak output power, gain and efficiency are, respectively, 290kW, 50dB and 11%.