A power supply device having an overvoltage cutoff function, an image display device, and a method of cutting off overvoltage are provided. The power supply device includes a switch unit which cuts off a power supply to the power supply device; and an overvoltage sensing unit which compares a voltage of the power supply device with a specified reference voltage, and if the voltage of the power supply device is higher than the specified reference voltage, generates an overvoltage cutoff signal to control the switch unit. The overvoltage cutoff signal is used to report a power supply cutoff state. Accordingly, the damage of the power supply device due to an unstable AC input voltage can be prevented and the user can be alerted to the overvoltage through the display of the power supply cutoff state.