Abstract: Electron optical system is an very important part of folded waveguide traveling wave tube. An electron optical system of a 220 GHz travelling wave tube is designed. The theoretical analysis is based on Pierce electron gun theory and synthesis iteration proposed by Vaughan, and numerical simulation of optical gun is completed by 3D software. The beam is focused by uniform permanent magnetic focusing system, and the uniform permanent magnet design is obtained by theoretical analysis and simulation. Then the uniform permanent magnet is machined and tested. The test result is in agreement with the design result. The electron gun and permanent magnetic focusing system are completely simulated, Which indicates that the beam transmission rate is 100%. The beam tunnel of the device is 0.3 mm in diameter over a length of 31 mm. The electron optical system is machined and tested, and the test result shows the beam transmission rate is 91% at voltage of 20 kV.