In this paper, a folded waveguide is adopted as a slow wave structure (SWS) of the extended interaction oscillator (EIO). An EIO with frequency 105 GHz in sub-Terahertz band is studied in detail, including the dispersion relation and the impedance of the SWS, and the variation of starting current with the period number, which provides an effective way to reduce starting current. On this basis, from the PIC simulation studies are discussed the EIO beam-wave interaction and output characteristics under the operation modes of low current continuous wave and strong current pulse. Further, the tuning characteristic of the EIO is analyzed, showing that output power reaches 26-50 W and the corresponding frequency range is 105.26-105.31 GHz. By EDM technology two fabrication methods of the SWS are investigated and the transmission properties of the SWS and the window are tested indicating that they are in good agreement with the simulation results.