A power detecting device, a power supply device using the same, and a reference voltage generator are provided. The power detecting device adapted to detect a power voltage of a display device includes a bandgap voltage generating circuit, a voltage regulating circuit, and a power-on reset circuit. The bandgap voltage generating circuit provides a reference voltage via an output terminal thereof. The voltage regulating circuit and the power-on reset circuit are coupled to the output terminal of the bandgap voltage generating circuit. When the power voltage doesn't reach a threshold voltage, the voltage regulating circuit increases the reference voltage referred by the power-on reset circuit. When the power voltage reaches the reference voltage, the power-on reset circuit generates a reset signal to reset the display device. Therefore, when the power voltage doesn't reach a stable, the power-on reset circuit will not be incorrectly started by increasing the reference voltage.