A power down circuit and a power down method are provided. The power down circuit includes a holding circuit and a comparator. The holding circuit is coupled to receive a first power signal. The holding circuit is configured to hold the first power signal to generate a second power signal. The comparator is coupled to the holding circuit. The comparator is configured to compare the first power signal and the second power signal to determine whether the first power signal is dropped to be lower than a first threshold voltage.