In this paper, the electron optical system for 140 GHz folded waveguide travelling wave tube (FTWT) is designed. The electron beam considering the thermal velocity effect in periodic permanent magnetic (PPM) focusing system is simulated and optimized by using the particle simulation software Opera-3D. The magnetic field is optimized to improve the matching effect between electron beam and PPM. The current of electron optical system is 60 mA when the anode voltage is 20 kV, and the simulation shows that the transmission efficiency is improved to 99.9%. In the experiment of sample tube, the parameters of the electron gun are in agreement with the design results. When the travelling wave tube (TWT) works in DC mode, about 97.2% of the electron beam reaches the collector.