A megawatt-level subterahertz surface wave oscillator(SWO) is proposed to obtain high conversion efficiency by using separated overmoded slow wave structures(SWSs). Aiming at the repetitive operation and practical applications, the device driven by electron beam with modest energy and current is theoretically analyzed and verified. Then,the functions of the two SWS sections and the effect of the drift tube are investigated by using a particle-in-cell code to reveal how the proposed device achieves high efficiency. The mode analysis of the beam-wave interaction region in the device is also carried out, and the results indicate that multi-modes participate in the premodulation of the electron beam in the first SWS section, while the TM01 mode surface wave is successfully and dominantly excited and amplified in the second SWS section. Finally, a typical simulation result demonstrates that at a beam energy of 313 ke V, beam current of 1.13 k A, and guiding magnetic field of above 3.5 T, a high-power subterahertz wave is obtained with an output power of about 70 MW at frequency 146.3 GHz, corresponding to the conversion efficiency of 20%. Compared with the results of the previous subterahertz overmoded SWOs with integral SWS and similar beam parameters, the efficiency increases almost 50% in the proposed device. A megawatt-level subterahertz surface wave oscillator (SWO) is proposed to obtain high conversion efficiency by using separated overmoded slow wave structures (SWSs). Aiming at the repetitive operation and practical applications, the device driven by electron beam with modest energy and current is theoretically analyzed and verified. Then, the functions of the two SWS sections and the effect of the drift tube are investigated by using a particle-in-cell code to reveal how the proposed device achieves high efficiency. The mode analysis of the beam-wave interaction region in the device is also carried out, and the results that that multi-modes participate in the premodulation of the electron beam in the first SWS section, while the TM01 mode surface wave is successfully and dominantly excited and amplified in the second SWS section . Finally, a typical simulation result demonstrates that at beam energy of 313 ke V, beam current of 1.13 k A, and guiding magnetic field of 3.5 T, a Compared with the results of the previous subterahertz overmoded SWOs with integral SWS and similar beam parameters, the increase increases almost 50% in the proposed device.