Staggered double vane traveling wave tubes have been given a lot of attentions in recent years due to its high power capacity and easy fabrication. However, the loss seriously limits the performance of traveling wave tube with the increasing of frequency, especially in THz wave band. In this paper, a more practical design about staggered double vane structure is proposed with the consideration of the loss and fillets caused by fabrication. The simulation results indicate that the tube with uniform period slow wave structures can obtain over 5W output power in the frequency range from 320GHz to 342GHz. The method of phase-velocity taper is used to enhance the output power and the simulation results show an obvious improvement of the power with more than 28% in the operating wave band. Based on these, the experiment of the fabricated high frequency system with filleted staggered double vane slow wave structure is carried out. The tested S21 of pillbox window is above -2.1 dB in the frequency range from 330GHz to 360GHz and VSWR (voltage standing wave ratio) is below 1.35 in the frequency range from 334GHz to 355GHz. And the tested VSWR of the high frequency system including the pillbox window is below 2 in the frequency range from 335GHz to 344GHz, which matches with the simulation results.