A method, a system and an authentication server for realizing a secure assignment of a DHCP address are disclosed. The method includes: sending a DHCP Discovery message via an access network; obtaining the identification information of the DHCP client and performing an authenticating to the DHCP client based on the identification information; and only assigning the address to the DHCP client has passed the authentication. Therefore, in the present invention, access authentication may be performed on a subscriber according to location information, and IP address is only assigned to the valid subscriber and terminal. Therefore, the security of the address assignment in DHCP mode may be enhanced greatly. Moreover, in the present invention, addresses may be managed by an AAA server unitedly, or the addresses may be assigned after being authenticated by the AAA server successfully.