Abstract: A high-efficient relativistic backward wave oscillator with a single-step change in the couple impedance has been designed and analyzed in the paper. An electromagnetic particle-in-cell code is being used to investigate the nonlinear beam-wave interaction in the device and other design optimization issues. The simulation results show that the device can generate 800MW peak power microwave at (9.16±0.03) GHz in the TM01 mode driven by 500kV , 5.5kA electron beams. The peak power efficiency is about 30%. The device optimum efficiency can be more up to 38% when a section of smooth drift tube lying between cut-off waveguide and slow wave structure is in the optimum condition.