Abstract: Increasing the single pulse energy is always one of the development targets in high power microwave (HPM) domain. Pulse shortening should be emphatically focused on and solved. Combining the classical theory and particle-in-cell simulation, a long-pulse C-band relativistic backward wave oscillator (RBWO) was designed and optimized. Using the space charge effect of the intense relativistic electron beam, the maximum field strength on the electrodynamic structure surface was no more than 700 kV/cm with about 3 GW output. Further, the related experiment was carried out based on the 700L high voltage pulse generator in our laboratory. As demonstrated by the studies, pulse shortening of the RBWO can be effectively suppressed at about 3 GW level through rational design. With a 760 keV, 9.0 kA beam in the experiment, a 2.8 GW, 4.23 GHz microwave was obtained in the far-field measurement. The pulse width was about 101 ns and the device efficiency was 41%. The experimental result agrees well with that obtained in the simulation.