The software startup and shutdown function of the power supply is achieved through the PW-OK circuit. During standby, PW-OK outputs a zero level power self-test signal to the host, and the host stops working and is in a standby state. After controlled startup, PW-OK delays for several hundred milliseconds from 0 level to+5V after the output voltage of the switching power supply stabilizes, and outputs a high-level signal to the host. This signal is equivalent to the PG signal of the AT power supply. The host detects a signal that the PW-OK power supply is intact and starts the system. If there is a power outage or shutdown during the operation of the host, the PW-OK output signal disappears several hundred milliseconds earlier than the ATX switch power supply+5V output voltage, notifying the host to trigger the system to automatically shut down before the power outage to prevent the hard disk head from not being able to move to the landing area and scratching the hard disk in case of sudden power outage.