The disclosure provides a method for updating an air interface key, a core network node and user equipment. The update method comprises: a core network node receives a relocation indication message, wherein the relocation indication message is configured to indicate that the User Equipment (UE) is about to relocate to a target Radio Network Controller (RNC) from a source RNC (Step S602); the core network node uses the key parameter to calculate a next hop enhanced key (Step S604); and then the core network node sends the next hop enhanced key to the target RNC (Step S606). Through the disclosure, the forward security of the user is guaranteed, and thus the communication security of the radio access system is improved.