The invention relates to a method and a system for realizing a secure assignment of DHCP address. The core of the invention is DHCP (Dynamic Host Configuration Protocol) client via the access network sends DHCP discovery message the access network side obtaining the port information of the DHCP client identification information and performing authentication based on the identification information at last the DHCP server only through authentication of the DHCP client allocation address information. Therefore the invention can perform access authentication to user according to the position information and is only a legal user distributing the IP address to user terminal so as to greatly enhance the security of distributing address through DHCP mode. And it can uniform the address managed by the AAA server or authentication by the AAA server succeeds after distributing address according to the present invention.