A backlight driving system driving a plurality of lamp groups comprises a phase controller, a pulse width module (PWM) controller, a plurality of power stages, a plurality of transformers and a plurality of switch circuits. The phase controller generates a plurality of phase signals. The PWM controller generates PWM signals. The power stages receive and convert direct current (DC) signals from an external power source to alternative current (AC) signals under the control of the PWM signals. The transformers receive and boost up the AC signals from the power stages to drive the lamp groups. The switch circuits alternatively transmit the DC power signals from the external power source and the PWM signals from the PWM controller to the power stages so as to control outputs of the power stages.