Abstract: With the increase of output microwave power, the internal field breakdown in a relativistic backward-wave oscillator (RBWO) gets gradually severe. The plasma generated by the field breakdown would lower the attainable output power and pulse width, which greatly limits the single pulse energy. Using a 3-dimensional particle-in-cell (PIC) simulation, the models involving single or several spot breakdown in the reflector, extractor and slow wave structure are built. The plasma effects existing in different locations or having different density are recognized. As demonstrated in the simulation, the output microwave power decreases rapidly as the plasma density increases, breakdown simultaneously occurring at several locations would result in pulse shortening more intensely in contrast to a single spot breakdown, and the easiest plasma effect exists in the breakdown of the reflector.