A multi-lamp driving system includes a pulse width modulation (PWM) controller, transformers, a current difference detection circuit, a lighting detection circuit, a frequency scanning detection circuit, a frequency regulating circuit, and a duty cycle regulating circuit. The current difference detection circuit detects difference among current flowing through lamps to determine if the current fluctuates. The lighting detection circuit determines if the lamps are lit according to the current, and generates a lighting indication signal after the lamps are lit. The frequency scanning detection circuit determines if the multi-lamp driving system is in a frequency scanning process according to the lighting indication signal, and generates a frequency scanning indication signal if the multi-lamp driving system is in the frequency scanning process. The duty cycle regulating circuit regulates duty cycles of the PWM signals upon the condition that the current fluctuates and the multi-lamp driving system is in the frequency scanning process.