In order to reduce the size of low-frequency high-power microwave instruments and realize the permanent magnet packaging for the instruments, a compact S-band relativistic backward wave oscillator is designed. Firstly, the dispersion equation of coaxial double grid slow wave structure is analyzed. Furthermore, the dispersion properties and distribution of electric field in the coaxial double grid slow wave structure are studied, making it the chosen high-frequency interaction region structure for the instrument. Ultimately, the instrument model design and experiments are carried out. The experimental results show that the instrument’s output microwave spectrum is pure, and both its radial and axial dimensions are notably decreased in comparison to the traditional S-band microwave instrument.