Abstract: This paper describes a method about the design of the transition region of the electron beam from partially shielded cathode with uniform and periodic magnetic focusing field. It is necessary to use paraxial ray equation in non-equipotential space for the electron optics system in which anode potential is not the same as helix potential or for the electron optics system in which anode potential is the same as helix potential, but the perveance is high, thus the anode hole influence is strong. Some idea about the problem of coincidence of electron trajectory in electron gun with magnetic flux line for the electron optics system using partially shielded cathode with periodic magnetic focusing field is presented.