The present invention relates to mobile communications technologies, and discloses a method, device, and system for pre-authentication. A pre-authentication device receives a pre-authentication message that carries a pre-authentication option; determines a Mobile Node (MN) to be pre-authenticated according to the pre-authentication message; sends an authentication request message to an Authentication, Authorization and Accounting (AAA) server to request authentication of the MN; receives an authentication response message that is sent by the AAA server, where the authentication response message carries a pre-authentication key used between a Candidate Authenticator (CA) and the MN; and sends the pre-authentication key to the MN. Through implementation of the present invention, the pre-authentication key is obtained before the MN switches to the CA. In this way, security of communication is enhanced, and delay of switching and authentication is shortened.