A kind of pillbox output window for 0.22 THz folded waveguide traveling-wave tubes is designed. By through theoretical analysis and numerical calculation in CST2010 and Ansoft HFSS11，this new kind of pillbox output window is verified. This pillbox output window adopts non-standard rectangular waveguide which can not only lower the difficulty of processing，but also increase the bandwidth. The results indicate that，the bandwidth can reach about 0.02 THz，from 0.21 THz to 0.23 THz，under the reflection coefficient?0.08. Some key dimensions which have significant effect on the pillbox output windows are also analyzed，and the tolerance in processing is given. The pillbox output window is validated through fabrication and tests. It proves out that the output window can be used for 0.22 THz folded waveguide traveling-wave tubes.