Abstract: A physical model about relativistic backward wave oscillator,based on the experimental system built in University of Electronic Science and Technology of China is supposed.The model includes a slow-wave structure consisting of an axially symmetric,cylindrical waveguide with sinusoidally rippled wall,and an annular electron beam of finite thickness going through the waveguide.The dispersion equation is derived.By numerical computation,BWO's bandwidth,growth rate and electron tuning ability affected by geometry parameters of slow wave structure,such as ripple's depth and length of ripple's period are analyzed.Some useful results for designing BWO are achieved.