By using a simplified electrostatic sphere on core diode model (SOC) the relative spherical aberration coefficients Cr of the magnetic field superimposed field emission gun (MFFEG) were numerically calculated. Here Cr is the objective side spherical aberration coefficient nomalized with respect to the spherical aberration coefficient in the absence of magnetic field, it provides a measure of the additional factor by which the electron source size is degraded as a result of the superimposition of the magnetic focusing field. The position of the real Image (the electron probe) could be adjusted easily by changing the exciting strength of the magnetic field. The Cr of bell shaped magnetic field has the least value among these 6 theoretical fields. Calculation suggested that the spherical aberration of this MFFEG system could be less than a pure field emission gun followed by a separate magnetic condenser lens. As an illustration, the MFFEG system with 10 mm magnetic field width could produce a beam crossover of 1 μm diameter (limited by spherical abberation) at 200 mm from the cathode, leaving a large free space around the specimen, for surface current density of 1.0×10 10 A/m2, the current into the crossover would be approximately 8 μA.