Is it a protocol of flooding to find the MAC address? Is it a protocol of flooding to find the MAC address? As we know, if the switch can not find a MAC address, it will flooding to every switchport except the entry port to find the corresponding MAC address. does this method is a protocol?

No. A switch emulates a repeater hub when it doesn't know the destination's port. However, in practically all cases the switch does know the destination port due to the destination having transmitted a frame previously and the switch learning the source address (from any broadcast, an ARP response or the like).

The flooding doesn't help the switch learning unknown addresses, it's rather a workaround to make the bridge not break the layer-2 connectivity.