A method and system for generating and distributing a mobile IP key are provided. The method mainly includes the following steps. During the course of access validation or re-authorization and re-authentication, a sub-key between a mobile node (MN) and an authentication, authorization, and accounting (AAA) server is computed, and an authentication extension (AE) between the MN and the AAA server is computed according to the sub-key. The AE is carried in a mobile IP registration request or a binding update (BU) request initiated by the MN, a home agent (HA) requests the key from the AAA server according to the AE in the registration request or the BU request, and the AAA server distributes the key to the HA. By using the present invention, a PMIP-client can compute the AE between the MN and the AAA server (MN-AAA-AE) without the need of distributing the key between the MN and the AAA server.