A light emitting diode (LED) backlight driving system drives at least one LED array, and includes a boost power stage circuit, a controller, a multi-channel constant current driving circuit, a voltage dividing circuit, and an offset voltage generating circuit. The boost power stage circuit boosts direct current (DC) power signals to output driving voltage to drive the LED array. The multi-channel constant current driving circuit controls current flowing through the LED array, and outputs regulating voltage to the controller to regulate the driving voltage. The voltage dividing circuit divides the driving voltage to generate feedback voltage to send to the controller. The offset voltage generating circuit generates offset voltage to modify the feedback voltage according to at least one mode selection signal. The controller controls the driving voltage according to the modified feedback voltage and the regulating voltage.