A high voltage charging circuit includes an activation control circuit, an inactivation control circuit and a power transistor. The activation control circuit is used for controlling an ON time period of the power transistor to enable the transformer to store excited-magnetic energy; the inactivation control circuit is used for controlling an OFF time period of the power transistor to make the transformer to release the excited-magnetic energy and use the excited-magnetic energy to charge the high voltage capacitor. Since the user can select a smaller Lp to keep constant the maximum value IP,max for the primary side current IP, the period tON can be made smaller as well, which means that a transformer 14 with a smaller volume can be utilized to reduce the entire size of the high voltage charging circuit.