There will be an electrostatic field between cathode grid and anode in directly heated electron guns. This electrostatic field has a primary convergence effect on the beam current. The change of distance between anode and cathode

and the change of grid and anode shape will affect this electrostatic field and ultimately affect beam quality. Aiming at the problem that the electrostatic field and the movement track of electron are both difficult to measure directly by conventional method

this paper built a 1 ∶ 1 3D model and simulated the whole process of beam generation by using the simulation software CST particlestudio. By using the single variable method and analyzing the date

we obtain electrostatic field distribution

clear electron movement track under different conditions and graph of each variable parameter and beam current energy density at the focus position. Finally

the best structure size for beam quality is obtained. Then we took actual welding test on aluminum alloy plate with optimized electron gun structure. The welding seams we got are better than the original electron gun structure

which proves the simulation result is correct. This method provides a basis for the improvement of the beam quality of the existing electron guns and future design of the electron guns.