Abstract: The cathode position and focusing current of the electron gun can affect the focal length and spot morphology of the electron beam, thereby affecting the quality of additive manufacturing parts. Firstly, based on the actual electron gun structure, a 1∶1 three-dimensional electron gun physical model and finite element analysis model were established, and simulation was conducted using the particle studio module of CST software. The effects of focusing current and cathode position on the focal position and spot morphology of the electron beam were studied. The simulation results show that the current gathered is 460 mA, and when the distance between the cathode's lower surface and the gate's upper surface is 1.0 mm, the diameter of the electron beam spot at the working plane is 0.61 mm. The electron distribution is more uniform, and the shape is more circular. An electron beam selective melting experiment was conducted, and the preheating and melting forming of titanium alloy powder were successfully achieved. The experimental results show that simulation has a certain guiding role in the design of the electron gun; it can meet the requirements for molding at an 800 mm work plane.