A novel slow-wave structure called ridge-vane-loaded folded waveguide combined with pencil electron beam has been proposed for millimeter-wave traveling-wave tubes. The high-frequency characteristics including the dispersion properties and the interaction impedance of this kind of structure have been analyzed by means of an equivalent circuit based on the transmission-line cascading network. The theoretical results agree well with those obtained by the 3-D electromagnetic high-frequency simulation software. The nonlinear interaction between the electron beam and the electromagnetic field is investigated with the help of the Computer Simulation Technology Particle Studio 3-D particle-in-cell (PIC) code. The PIC simulation results revealed that this kind of structure could produce about 56 W of peak output power at 0.22-THz when the cathode voltage and current of the pencil electron beam are set to 14.65 kV and 50 mA, respectively. Moreover, the maximum gain and interaction efficiency can reach 37.5 dB and 7.7% at 0.22 THz, respectively. A simulated 3-dB bandwidth of ~ 30 GHz is demonstrated.