The scattering matrix for the cavity with abrupt changes in radius in the gyroklystron was built with the mode-matching technique. The variations of the resonance characteristic of the cavity modes with the length and radius of the cavity have been studied. The cavities for input, bunching and output of the Ka-band TE01 mode in the gyroklystron have been designed and fabricated.The designed amplifier, as indicated by the PIC simulation, was expected to produce 390 kW peak output power, saturated gains of 42.9 dB and 50.6% efficiency at center frequency 34 GHz and 70kV, 11A annular electron beam. The full width half maximum (FWHM) bandwidth is greater than 360 MHz. The hot test shows that the fabricated gyroklystron amplifier has actually produced 301 kW peak output power, 39.1% efficiency and saturated gains of 41.8 dB at center frequency 34 GHz and 70 kV ,11 A annular electron beam . The full width half maximum (FWHM) bandwidth is greater than 285 MHz.