A Ku-band backward wave oscillator with low magnetic field is designed in order to realize the High Power Microwave(HPM) system miniaturization, according to the basic theory of traditional Relativistic Backward-Wave Oscillator(RBWO) with low magnetic field. The effects of beam voltage, beam current and the guiding magnetic field on the output power are analyzed. The structure is optimized by using particle simulation(PIC). A microwaves output power of 1.0 GW with the frequency of 13.1 GHz is obtained under the guiding magnetic field of 0.4 T, the diode voltage of 600 kV and the beam current of 7 kA. The preliminary experiment is performed at low magnetic field condition. Under a guiding magnetic field of 0.4 T, the microwave power of 850 MW with 13.05 GHz frequency and 24 ns pulse width is obtained. The experiment has laid a good foundation to realize the GW microwave output with lower magnetic field.