A power control device includes a comparison unit, a control unit and a switch unit. The comparison unit electrically connects to a first power rail and a second power rail, respectively, and generates a comparing signal according to a reference signal, a first voltage level and a second voltage level. The control unit electrically connects to the comparison unit for receiving the comparing signal. Then, the control unit generates a control signal based on the comparing signal. The switch unit electrically connects to the control unit, and is on/off based on the control signal, so as to connect/disconnect the first power rail and the second power rail.