A new type of high power microwave source operated at Jow magnetic Geld is proposed and studied by the particle-in-cell (PIC) method. An oversized uniform backward-wave-oscillator-like structure is connected to a tapered slow-wave structure by a resonant cavity. In this device, the electron beam current is effectively used to yield microwaves with high efficiency, and the mode is locked in a wide range of diode voltage to reduce the requirement to the voltage wave form. The PIC simulation results show that a peat microwave power of 2.1 GW (averaged 1.1 GW) with a pure TMoi mode at 10. 7 GHz is radiated with 4.1 GW input beam power, the frequency remains approximately the same when the diode voltage changed from 300 kV to 700 kV.