An embodiment of the present invention provides a method for allocating a public IP address, including: receiving, by a CGN, a user on-line packet, and judging whether an idle public IP address exists; when the public IP address pool doesn't have an idle public IP address, constructing and sending, a DHCP Request packet to a DHCP Server, and requesting obtaining a public IP address set; and allocating, a first public IP address in the public IP address set to the user. Embodiments of the present invention further provide an apparatus and a system for allocating a public IP address. Unified management and allocation of the public IP address of the CGN device can be implemented through the method, the apparatus, and the system provided in the embodiments of the present invention. The utilization ratio of the public IP address of the CGN device can be effectively improved.