Abstract: Operating principle of series diode is introduced and an equivalent circuit of series diode is built to analyze its working process. Circuit simulation is done to find the influencing factors of series diode working process. Voltage dividing of series diode includes four phases: first, voltages of each diode are divided according to capacitances of each diode. Second, voltages of each diode are divided according to the first diode impendence and second gap capacitance. Third, Voltage dividing enters into a dynamic balancing phase relating to all the parameters after both cathodes emitting electrons. Finally, Voltage dividing is determined by the impendence of each diode. Capacitances and vacuum breakdown voltages of diodes are the major influencing factors of voltage dividing in the primary stage, and impedance variation and vacuum breakdown time of each gap have important impact on voltage dividing in the dynamic balancing phase.