A pre-authentication method and an authentication system related to the mobile communications field are disclosed. The pre-authentication method includes: when a mobile node (MN) enters a visited network other than a home network, the MN obtains the identity information of the visited network, selects, according to the identity information of the visited network, a first pre-auth-key-file corresponding to the visited network and a first ticket corresponding to the visited network, where the first ticket carries the first pre-auth-key-file, and authenticates the visited authentication, authorization and accounting (VAAA) server according to the first pre-auth-key-file.