Key generation method and system when a tracking area is updated

The present invention provides a key generation method when a tracking area is updated, wherein during the process of updating a tracking area, when the active flag is in valid state, a user equipment and a mobility management entity generate an evolved NodeB key according to an access security management entity key and a downlink non-access layer counter, or generate the evolved NodeB key according to the access security management entity key and a random number generated by the mobility management entity, or generate the evolved NodeB key according to the access security management entity key and a constant number. The present invention also provides a key generation system when a tracking area is updated and a mobility management entity and user equipment for generating a key when a tracking area is updated. By using the present invention, the evolved NodeB key can be generated when the active flag is in valid state, and the coherence of the evolved NodeB keys generated by the Mobility Management Entity (MME) and the User Equipment (UE) can be ensured.