The focusing characteristics of electron guns sufficiently affect the quality of electron beam welding. At present, no systematic theoretical method calculates the beam waist and beam spot parameters that reflect the focusing characteristics of electron gun. Aiming at this problem, this paper contains a theoretical analysis of the relationship among the potential on the electron gun axis, the diameter of beam waist, and the trajectory of charged particles that pass through the focusing coil. A simulation model based on a F19-01 high-voltage electron gun for this study is developed using the CST simulation platform. The model simulates the variation of electron beam waist and beam spot diameter under different electrostatic focusing and magnetic focusing parameters. Results of a single-parameter study show that an increase in the cathode-to-anode distance leads to the increment in the beam spot diameter, whereas an increase in the bias cup aperture, as well as increases in anode aperture and focusing current, cause its decrease. An orthogonal test confirms that the aspect ratio of the electron beam welding seam under the same power can be improved effectively by selecting the cathode-to-anode distance of 28.4 mm, the bias cup aperture of 6 mm, and the anode aperture of 10 mm. Thus, the experimental data prove the correctness of the theoretical and simulation analyses.