A MIP system and a method for updating an HA-RK are disclosed. The AAA server generates and delivers a new HA-RK before expiry of the old HA-RK, thus eliminating the time gap between expiry of the old HA-RK and obtaining of the new HA-RK and making the MIP registration seamless. In the system, if the remaining lifetime of the old HA-RK is less than or equal to the lifecycle of the MSK in the EAP process, a new HA-RK is delivered; otherwise, no new HA-RK needs to be delivered. If both a new HA-RK and an old HA-RK are valid on the network entity at a time, then only the old HA-RK applies and the new HA-RK is not active until expiry of the old HA-RK. Alternatively, both the new HA-RK and the old HA-RK are active concurrently, and are differentiated by an SPI.