Abstract: The resonant cavity reflector has been used in the backward wave oscillators for producing multi-gigawatt power and operating at low magnetic field. The reflection characteristics of the “cold ” resonant cavity reflector is analyzed by means of the mode-matching method, and the scattering matrix of the reflector is derived. The electric field distribution in the reflector is studied by means of numerical simulation under total reflection condition. The results indicate that with suitable inner and outer radius, high reflection can be obtained with short cavity length, and the cavity length does not affect the reflectivity remarkably.