Abstract: An 1 2/2D relativistic electromagnetic plasma particle simulation code is made. The system this code describes is cylindrical symmetric, and has non-periodic boundary. An intense REB injects into a waveguide with one end closed and the other opened. Produced virtual cathode oscillation and excited high power microwave are studied. Virtual cathode oscillation frequency is in agreement with the empiric expression (1～√2π)ω_peb. Up to 10% of microwave efficiency is gained. In addition, dependence of radiation frequency and efficiency on external magnetic field, electron beam intensity, electron energy and beam radius is studied through simulation calculation.