The current-voltage characteristic, the trajectory of the "boundary" electron and the dependence of "anode spot" sizes on the magnetic field induction have been calculated for the field emission spherical diode based on the metal field emission cathode. The effect of the space charge in a cathode-anode vacuum gap on the emission regularities and variations of its configuration under the action of the longitudinal external magnetic field applied for the beam focusing have been taken into account. Analysis of the results of the calculation and numerical experiment show a good agreement between theory and experiment.