Internal field emission breakdown in the electro-dynamic structures of high-power microwave devices can seriously limit the devices’ output power and pulse duration. So an over-sized backward wave oscillator (BWO) is developed to increase the diameter of the electro-dynamic structure beyond the cut-off radius, and reduce these internal fields to levels, which are below critical breakdown levels. As a typical high power microwave (HPM) device, the oversized BWO is widely used and investigated. But some interaction phenomena between the beam and the microwave field in the device are not clearly understood because the beam-loaded effect is so obvious. And the physical process for the interaction is also considered to be complicated. Here as an oscillator, the feedback process is very important in the microwave device, which includes the oversized BWO. So the interaction process between the beam and the oversized BWO is explored from the feed back process instead of the field in the device. Then the physical mechanism for the feedback process in the oversized BWO is explored both in theoretical investigation and in particle-in-cell simulation. And the equivalent circuit is established for such a purpose. The mode control mechanism is explored based on the equivalent circuit. Finally an over-sized backward wave oscillator with rectangular profile corrugations is designed to produce TM ₀₁high power microwave radiation without mode-competition. An RF power of 7.9 GW at a frequency of 8.68 GHz is obtained in the particle in cell simulation driven by the beam with a beam voltage of 1 MW and a current of 20 kA, and the corresponding efficiency is 39.5%.