Abstract: Distribution of electromagnetic field in the space of Htype RF ion source is researched theoretically. Threedimensional vector graphs of electric field intensity and magnetic induction density in the space of RF ion source are calculated and plotted by Mafia software. The conclusion that axial electric field intensity is more important during the breakdown of RF ion source is made out by comparing it and azimuthal electric field intensity before breakdown, and then, the breakdown criterion of RF ion source is calculated, which is reasonably in agreement with experimental data.