In order to study the influence factors on the performance of FWG-TWT(Folded Wave- Guide Traveling-Wave Tube)，a 0.22 THz FWG-TWT was designed and simulated，some important elements，including beam voltage，current，input signal power，structure periods number，material conductivity，guide magnet，energy spread，and angular spread of emission electrons，were numerically studied and discussed. The simulation results indicate that，there is an optimal beam-voltage range；the gain can be improved by increasing electric current；the input signal power is not the higher the better，and the number of structure periods is not the more the better；higher conductivity and surface finish of the material could increase the gain of the device；the guide magnet will be high enough when the electron beam can be constrained；and improving electron quality by controlling electron energy and angular spread can increase the gain of the device. The FWG-TWT was also simulated by using a massively 3-D fully electromagnetic PIC code—NEPTUNE，programmed by the authors，the physical images of beam-wave interaction were given，and the simulated results agreed well to other software. At the same time，the self-oscillation phenomena caused by port-reflection were studied and discussed，and some suggestions were put forward.