In this paper, a 0.14 THz relativistic backward-wave oscillator (RBWO) filled with neutral argon gas is simulated by using the UNIPIC code. The effects of plasma on the output power, frequency, and the start-oscillation time of the RBWO filled with different-pressure gases are studied. The simulation result shows that the background plasma can cause the pulse-shortening and new frequency appearing in the 0.14 THz RBWO. Appropriate plasma infusion can reduce the start-oscillation time and increase the output power of 0.14 THz RBWO.