A driving circuit for an LED backlight system is disclosed. The driving circuit includes an input voltage, an input resistor, an operational amplifier, a first transistor and a current calculation unit. The operational amplifier has a positive input terminal electrically connected to the input voltage through the input resistor, and an output terminal electrically connected to its negative input terminal thorough a feedback network. The first transistor is utilized for draining a reference current to control an output voltage of the operational amplifier according to the input voltage and the input resistor. The current calculation unit is utilized for generating a plurality of working currents proportional to the reference current to drive a plurality of LED strings according to the output voltage of the operational amplifier.