Abstract: By solving Laplace’s equation with the first class boundary condition, the analytical expression of the axial potential distribution in a typical triode immersion objective was achieved. Based on the related expression, effect of the geometrical parameters and potential ones on the focusing properties of the electrostatic lens is illustrated. The optained conclusions are that ununiformity of the axial electric field directly results in the relatively weakened focusing field, together with the improvement of particles’ temporal and spatial dispersion. The achieved modulating means are as follows: augment of the potential value and aperture size of the modulating electrode, increase of the intervals between modulating electrode and accelerating one, decrease of that of cathode and the form