It is provided a method and system for multi-access authentication in Next Generation Network (NGN). A network side authentication center (NSAC) generates an authentication vector after receiving, from a user terminal (UT), UT information including subscription information and multi-access information of the UT; after receiving an authentication request including authentication information from the NSAC, the UT performs authentication on the network side, generates keying material and network side authentication information (NSAI) upon successful authentication, and sends the NSAI to the NSAC, which performs authentication on the NSAI using the authentication vector, generates keying material according to the multi-access information of the UT upon successful authentication, and informs an access forwarding functional module (AFFM) of the keying material; the AFFM encrypts and decrypts access service information of the UT according to the keying material. The efficiency with which the UT accesses the network is enhanced.