The invention provides a circuit system for driving a high-intensity discharging lamp, comprising a boosting circuit, an ignition coil circuit and a clamp circuit. The boosting circuit includes a first transformer and a first switch, in which the primary of the first transformer receives an input power, the secondary of the first transformer produces a boosting DC voltage, and the first switch is connected to the first transformer to control turning-on and turning-off of the first transformer. The ignition coil circuit is connected to the boosting circuit for converting the boosting DC voltage into a switching AC voltage to drive a load. The clamp circuit is connected to the boosting circuit and the ignition coil circuit for directing energy, reflected from the secondary of the first transformer to the primary of the first transformer, to the secondary of the first transformer as the first switch is turned off.