Poisson Superfish combined with General Particle Tracer was used to design the electrostatic focusing immersion lens for 100 keV ultra-fast electron diffraction (UED) system. The anode is a disk and cylindrical protruding structure, and also serves as the front electrode of the lens. The back electrode has a hollow double-cylinder structure, with the top cylinder located in the center of the circular top surface of the bottom cylinder. The top cylinder head is chamfered smoothly, and the cylindrical protrusion of the anode is aligned with the top cylinder head of the back electrode. Based on this structure, the farther away from the central axis, the greater the radial component of the electric field, and the better the electron pulse focusing effect. With an initial electron pulse radius of 50 μm, the immersion lens can achieve good convergence under conditions when the focusing voltage is -5 kV and the number of electrons is 1000 and when the focusing voltage is -10 kV and the number of electrons is 2000 and 10000 respectively.