The design of three sections phase velocity taper is put forward in 0.14 THz folded waveguide Traveling Wave Tubes(TWT) research. It can maintain beam-traveling wave velocity synchronization again in output section of interaction circuit and improve the beam operating efficiency. A structure parameter impacting on phase velocity change is found and optimized by slow wave structure dispersion equation. After optimizing design, a large signal program calculation is performed under operating voltage 14.95 kV and current 30 mA. Compared with the structure without phase velocity taper, the efficiency is improved by 9.13% in 140 GHz, and the output power in improved by 0.84 W. The efficiency is improved by 10.4% in 142 GHz, and the output power is improved by 0.88 W. -1 dB bandwidth is improved to 7 GHz from 5 GHz. The design improves the bandwidth and beam-traveling wave work efficiency in D band folded waveguide traveling wave tubes.