A W-band ridge-loaded folded waveguide traveling-wave tube (FWTWT) is modeled, and the nonlinear interaction between the electron beam and the electromagnetic field is investigated by utilizing a 3-D particle-in-cell (PIC) code, MAGIC. The process of beam-wave interaction for millimeter wave TWT is presented, including the longitudinal momentum of the electron, the averaged radiation power, and the influence of input power on output power. Compared with the conventional FWTWT at 95 GHz, the ridge-loaded FWTWT obtains a 29.3% higher output power and shortens 31.7% interaction distance. The radiation power and the gain of TWT are raised up in most of the working band covering from 85 to 97 GHz, indicating effectual enhancement of beam-wave interaction by loading ridge. Furthermore, the novel FWTWT works in 7-GHz bandwidth with output power varying only 1 dB throughout the band.