An exemplary light emitting diode (LED) driving system includes a direct current/direct current (DC/DC) converter, a detection circuit, a control circuit, a pulse width modulation (PWM) controller, and a current balance circuit. The DC/DC converter outputs a suitable direct current voltage to drive an LED array. The detection circuit detects cathode voltages of LED strings of the LED array. The control circuit generates and outputs a control signal to the PWM controller, and generates and outputs various adjusting signals. The current balance circuit adjusts current flowing through two of the LED strings, which have a minimum and a maximum detected cathode voltage, respectively. The current balance circuit includes switches. A related LED driving method is also provided.