Abstract: The magnetic lens, as a key component of the electron gun, greatly influences focusing performance, spot diameter, and welding quality through its structure. A physical model of the electron gun beam source and electromagnetic focusing structure is constructed based on the ZD-VEBW series high-voltage electron beam welding equipment. Utilizing the CST platform, simulations and analyses are performed to understand the effects of the inner diameter of the magnetic lenses' iron shells and the iron shell openings on the spot diameter. The ring baffle design facilitates adjustments to the inner diameter of the magnetic lens iron shell and the iron shell opening parameters. Welding experiments are conducted with varying focusing currents. These experiments show that optimal magnetic focusing characteristics, resulting in the smallest beam spot diameter and the largest weld depth-to-width ratio, can be achieved through a reasonable design of the magnetic lens structure parameters. The simulation results are validated by these experiments.