A new type of high power microwave device is developed based on bitron and backward wave oscillator. The device is composed of two parts: the modulation cavity and the extraction cavity (which is similar to slow wave structure). The modulation cavity acts as electron beam modulator and microwave reflector, which forms a microwave resonator in combination of the extraction cavity. The electron is modulated when it passes through the modulation cavity, and the high power microwave is generated when the modulated beam passes through the extraction cavity. An X-band high power microwave device is designed for a 20 GW accelerator, and the simulation results are frequency 8.25 GHz and output power 5.70 GW. Using superconducting magnet as guiding magnet, a microwave power of 5.20 GW at X-band (frequency (8.250.01)GHz) is obtained in single pulse mode. The radiation power is 5.06 GW when the repetition rate is 30 Hz, and the pulse length is 13.8 ns.