A design of the electro-optical system for D-band Travelling Wave Tubes (TWTs), including an electron gun and a Periodic Permanent Magnet(PPM) focusing system, is presented and verified. The electron gun utilizes the classical Pierce electron gun structure, the outer layer of the cathode emission surface is set with a cathode sleeve to suppress stray emission at the edge of the cathode, and a cylindrical control electrode is used instead of a conical control electrode, and a negative bias is added to the focusing electrode to adjust the compression state of the electron injection. The electron gun can provide a beam current 57 mA with the beam waist radius of 0.068 mm and the shooting distance of 14.9 mm at the beam voltage of 1.9 kV. A sleeve tube is attached on the cathode to block the electron emission from the edge of the cathode. A cylindrical focusing electrode is used in the electron gun for adjusting the beam focusing state. To stably focus and transmit the electron beam in the beam tunnel of 0.15 mm radius, a PPM focusing system is employed. The peak magnetic field is 2.9 times the Brillouin magnetic field to increase the rigidity of electron injection. The simulated result shows the fluctuation radius of the electron beam is less than 0.1 mm. In order to validate the effectiveness of the D-band TWT electro-optical system, the electron gun and the PPM focusing system have been machined and assembled. The experimental results show the beam current is 49.83 mA, and the collector current is 49.6 mA. The beam transmission rate of 99.5% are obtained by carefully adjusting the PPM system, which achieves the design goal.