A new type of high power microwave device is developed as a quasi-bitron. The device is simple and composes of two parts, the modulation cavity and the extraction cavity, with a coupling between the cavities. The structure has two working modes, one of them is the working mode, and competition between modes can be easily avoided even when the RF field in the device is very strong compared with the BWO (backward wave oscillator). Besides the theoretical analysis, the model has been realized on 2.5 PIC (particle in cell) simulation code. A 9.42GHz bitron oscillator is designed and its theoretical efficiency is 29.4%. The simulation shows the device achieves the efficiency 28% when the beam is 800 kV and 10 kA, and outputs RF power of 2.25GW. The experiment was also carried out on Sinus-700 accelerator. The measured microwave power is 2.44GW at the frequency 9.4GHz.