This application relates to the field of communications technologies. This application discloses a method, an apparatus, a network device, and a system for releasing an IP (Internet protocol) address, to resolve a problem that a time at which the IP address is released is not precise. The method includes: determining, by the first network device, that a PDU session anchor needs to be relocated from a first user plane function entity, where the first user plane function entity is the PDU session anchor of a first PDU session of a terminal; determining, by the first network device, that a first IP address of the terminal is no longer used, where the first IP address is an IP address used by the terminal in the first PDU session; and releasing, by the first network device, the first IP address.