An S-band Relativistic Backward Wave Oscillator(RBWO) with lower magnetic field is designed in order to achieve the miniaturization of High-Power Microwave(HPM) systems. According to the characteristics of low magnetic field, the effects of slow wave structure, guided magnetic field, beam pressure, beam current, etc. on the output microwave are analyzed. The structure is optimized by using particle simulation software. On this basis, a low magnetic field S-band relativistic backward wave tube is designed, which has a guided magnetic field of 0.24 T, an electron beam voltage of 725 kV, a beam current of 6 kA, a frequency of 3.53 GHz, an output microwave power of 1.22 GW and a beam conversion efficiency of 27%. The preliminary experiment is performed on the accelerator platform with high current electron beam when the external magnetic field is 0.24 T. The microwave is output with average power of 1 GW, 3.58 GHz frequency and 90 ns pulse width. And a stability test at 1 Hz repetition frequency in 20 s is performed. The experimental results have laid a good foundation for realizing the relativistic backward wave tube of the permanent magnet packaging.