Abstract: By elaborately designing the slow wave structure (SWS), an oversized terahertz backward wave oscillator(BWO) with high output power and high beam-wave conversion efficiency is presented and the 2.5-D particle-in-cell simulations are carried out using a PIC code. Under the condition of 280 kV voltage and 320 A current, the output power of 130 MW at frequency of 121.8 GHz is achieved when the strength of the guiding magnetic field is 3 T, and the beam-wave power efficiency of 14.5 % is obtained.