This paper mainly discusses the pill-box window with sapphire piece for 0.14 THz folded waveguide traveling wave tubes. The dimension of the window is computed and optimized by HFSS. The VSWR is below 1.2 when the main structure parameters range at ±0.01 mm. Cold-test result shows the attenuation of the window test structure is about 0.7 dB during 0.135 THz-0.145 THz，which meets the practical requirements in 0.14 THz folded waveguide traveling wave tubes.