In the paper, the nonlinear theory of relativistic backward wave oscillator(RBWO) with variable couple impedance was built up, the relations of variable couple impedance RBWO efficiency with couple impedance change location and highlow couple impedance rate was numerically modeled and optimized using the nonlinear theory. A xband RBWO with a single step in the couple impedance has been designed and analyzed by use of an electromagnetic particleincell code and investigated experimentally, the preliminary experiment results show that the device efficiency is about twice of the uniform RBWO in the conditions of the same operating parameter.