According to the principle of electromagnetic lens, the size of the beam spot on the workpiece is related to the position and size of the electron beam waist. The change of the diameter of the grid hole and the position of the anode will cause the change of the electric field distribution, which will change the position and size of the electron beam waist, and eventually influence the size of the working beam spot. The electric field distribution and beam trajectory of the electron gun electrostatic lens are obtained by using the EMF simulation software CST. A single variable method was used to simulate the influence of the diameter of the grid hole and the position of anode on the position and size of the electron beam waist, while the intensity of beam was set to 100mA. The simulation results show that when the diameter of the grid hole is 2mm and the distance between the upper surface of anode and the lower end face is 15mm, the position of electron beam waist is highest and the radius is the smallest in the situation of that other parameters are the same. The welding test was carried out by the optimization parameter D=2mm and H=15mm. The welding seam with greater depth to width ratio was obtained. It is proved that the purpose of reducing the size of the working beam spot and optimizing the beam quality is reached.