For the purpose of coherent high power microwave combining based on Relativistic Backward Wave Oscillator(RBWO)，an optimal design of a low magnetic field RBWO is present. In order to decrease the requirement on the seed RF power for phase locking, the axial position of the intermediate bunch cavity of the RBWO is optimized to decrease the Q-value of the working mode. Particle In Cell(PIC) simulation shows that an input power of 20 MW is sufficient to lock the frequency and phase of the output. And the corresponding gain is higher than 20 dB. The result is also valuable for phase locking by modulated electron beams.