Abstract: The paper presents a three-mirror quasi-optical cavity (TMQOC) high power radiation source, which includes a column slow-wave grating and two spherical mirror reflectors. A TMQOC system has been designed on the basis of simulation and theory calculation. With a ring beam passing over the column grating surface, the TMQOC radiation source can output a power of 1.2 MW at W-band, under the condition of guiding magnetic field of 2.2 T, diode voltage of 557 kV, beam current of 1.8 kA with duration of 70 ns.