Abstract: The pre-pulse phenomena are studied by the simple theoretical analysis and the method of electro-circuit simulation in a PFL charging-up by MFCG or capacitors. It is shown that what affect the amplitude of pre-pulse voltage are the grounding inductance and the frequency of the charging loop circuit, which is determined by the secondary coil inductance of transformer and the capacitance of PFL. The parasitical oscillation with high frequency introduced by the grounding inductance doesn't change the peak value of pre-pulse voltage. The frequency of the charging loop circuit is low enough that the peak pre-pulse voltage is only on kV level as PFL is charged through a transformer. The experiments are carried out when the PFL is charged by capacitors through a pulse transformer in two cases tha