A method for defending a DHCP attack is provided. The method includes monitoring packets transmitted by a client terminal coupled to a target port, and determining, in IP addresses allocated for the client terminal coupled to the target port, the number of the IP addresses generating no packet traffic in a preset first time. And the method further includes if the number of the IP addresses generating no packet traffic reaches a preset first threshold, determining that the client terminal coupled to the target port has risk of attacking, and restricting DHCP service of the target port. A device for defending a DHCP attack is also provided.