To develop high frequency, broad bandwidth and low voltage traveling wave tube (TWT), the folded rectangular coaxial waveguide slow wave structure (FRCW-SWS) is proposed in this paper. Operating at overmoded condition, the proposed FRCW-SWS has relatively high operating frequency and acceptable transmission characteristics. A double ridge-loaded waveguide-to-coaxial-waveguide converter is designed and applied as the input/output coupler to incorporate the broadband characteristics of the FRCW-SWS. The hot performance of this overmoded FRCW-TWT at W-band is investigated using particle-in-cell (PIC) simulation. The simulation results indicate that the output power can be over 13.7 W in the frequency range of 76～110 GHz, when the length of the SWS, the beam voltage and beam current are 32 mm, 3230 V and 150mA, respectively. The maximal output power is about 27.4 W at the frequency of 108 GHz, corresponding to the RF efficiency of 5.65%.