A protection circuit, applied to a flash memory including a power supply pin, includes a capacitor and a switch. A power supply provides a reference voltage. The capacitor is electrically connected to the power supply pin and a ground point. The switch is electrically connected between the power supply pin and the power supply. When the reference voltage is higher than a threshold voltage, the switch is turned on, such that the reference voltage is inputted into the power pin via the switch. When the reference voltage is lower than the threshold voltage, the switch is turned off.