Abstract: As a novel slow wave structure, there is not a suitable non-linear analysis method for folded waveguide to achieve the accurate phase simulation, which is important to improve the electron efficiency of millimeter wave traveling-wave tubes. By using PIC Solver in CST PARTICLE STUDIO, the phase difference of the first time-harmonic components of the beam current and the electric field is adopted to describe the phase focusing in beam-wave interaction in FWG TWTs, and an analysis method of phase is proposed. Via comparison between normal structure without velocity taper and the structure with velocity taper in the output part, the physical mechanism of phase analysis in beam-wave interaction is illustrated and ana- lyzed in detail, and the electron efficiency is improved by 5. 13%, which validates that the method is right and practical.