Abstract: In order to reduce the volume and weight of the high power microwave (HPM) system, an S-band relativistic backward wave oscillator(RBWO) operating at low guiding magnetic field is designed. The operation magnetic field of the RBWO is lower than that of the electron cyclotron resonance, which will decrease the electron beam transmission efficiency and beam-wave interaction impedance. By increasing the distance between the electron beam and the inner wall of the device, the transmission efficiency of the electron beam can be improved. The deep slow-wave structure is used as extraction cavity to enhance the efficiency of beam-wave interaction. The shallow slow-wave structure in front of extraction cavity is adopted to ensure synchronization of electron velocity and microwave phase velocity. The high power microwave is acquired under low guiding magnetic field. Simulation shows that 670 MW output power with 25% efficiency is generated under a guiding magnetic field of 0.17 T, electron voltage of 435 kV and beam current of 6.5 kA. Compared to the conventional magnetic field system (B of 0.8 T), the radius of solenoid of the 0.17 T low magnetic field system applied to the RBWO is reduced by 20%, and the energy consumption is reduced by about 93.2%.