Abstract: RF breakdown of the metallic high frequency structures is an important reason of power decrease and pulse shortening; it is also an important limitation of the development of high power microwave (HPM) to higher power and longer pulse. The physical process of RF breakdown is pretty complicated, and the experiment research of RF breakdown is hard to conduct, thus simulation research becomes an important method. We simulated the influence of plasma on the performance of an X-band RBWO and a Ka-band RBWO by setting electron and ion emission from the surface of SWSs. The simulation results show that the formation of plasma in back SWSs will influence the performance of RBWO by influencing the electron beam. When the plasma consists of light ions and electrons, it will bring stronger influence on the beam wave interaction, leading to power decrease in a high degree. The breakdown plasma has a bigger influence on the Ka-band RBWO than X-band RBWO.