A method of driving an LED string is provided and includes steps of: (a) determining whether or not a detected voltage of a control terminal of a transistor is lower than a first reference voltage by a first comparator, if not, raising a voltage of the LED string by a power supply device and performing step (a), if yes, performing step (b); (b) determining whether or not the detected voltage is lower than a second reference voltage by a second comparator, if not, determining that a short circuit occurs, if yes, performing step (c); (c) increasing an output current of the LED string by an input current source; and (d) determining whether or not a current of the LED string reaches a maximum current that is adjustable by a light controller, if yes, determining that a short circuit occurs, if not, returning to step (c).