Abstract: This paper introduces the design method of the electron optical system for a 0.22 THz folded waveguide traveling wave tube. As the electron beam current is 10 mA and the beam channel radius of the tube is only about 0.1 mm, the PPM(period permanent magnet) focusing system needs to be carefully designed to transmit this beam current from the electron gun to the collector. A tree-electrode Pierce gun is adopted. The electrodes and PPM are optimized to obtain proper beam waist radius and high transmission efficiency. Maxwell is used to calculate the magnetic flux density on the beam line, and CST is used to simulate the beam transmission. The simulation shows that about 96% of the electron beam reaches the collector and is verified by experiments, which gets about 74% transmission efficiency without magnetic modification. After magnetic modification, the transmission efficiency is improved to 94%, and this tube is able to run in DC mode.