This paper discusses the basic theory of PIC-MCC method and its implementation in the software UNIPIC. Simulation of relativistic backward-wave oscillator (RBWO)filled with different kinds of neutral gas of different pressures is made by using UNIPIC. The effects of plasma on the propagation of electron beam and wave-beam interaction are discussed. The dependence of the output power，frequency，and the start-oscillation time of the RBWO on the kinds and pressures of the filled gases are presented. The simulation results show that the improvement of RBWO is mainly attributed to the produced ions，and that the low-energy electrons remaining in the tube is the key factor of oscillation-failure and pulse-shortening.