Abstract: The electrostatic focusing characteristics of electron guns affect the quality of electron beam welding.At present, there is no systematic theoretical method to calculate the beam waist parameters that reflect the electrostatic focusing characteristics of different types of electron guns. The mapping relationships between the potential distribution on the axis of the electron gun and the beam waist radius and position of the electron gun are both theoretically analyzed. The electrostatic focusing characteristics of the electron gun are explored. Based on the CST simulation platform,one electron gun is taken as an example. The excitation source parameters and boundary conditions are numerically set.And the electrostatic lens analysis model of an electron gun is constructed. Moreover, the beam waist positions and cross-sectional areas of the electron beam under different structural parameters are acquired. Combined with the numerical analysis results, the effects of different structural parameters on the on-axis potential are discussed. And the electrostatic lens analysis model of the electron gun is verified according to the numerical analysis results. The simulation results provide theoretical support for the design and optimization of the key parameters aiming at the electron gun.