Abstract: Firstly, an X-band backward-wave oscillator(BWO) with a low guiding magnetic field is designed, whose output microwave power of 1.24 GW at frequency of 9.1 GHz is achieved under the condition of 750 kV voltage and 5.5 kA current when the strength of the guiding magnetic field is 0.46 T. In order to lower the strength of the guiding magnetic field so as to actualize the permanent-magnet-packaging of the relativistic BWO (RBWO), special slow wave structure is designed to increase the quality factor Q of the device, simultaneously increase the coupling impedance between the electron beam and the slow wave structure. Then a 0.46 T permanent magnet is developed, which is 48 cm long and 116 kg in weight. At last, experiment is carried out to investigate the RBWO with the permanent magnet, a 900 MW output power at 9.08 GHz frequency is obtained when the device is operating in 20 Hz repetitive rate for 1 s, while a 940 MW output power is obtained in single pulse condition.