The invention discloses a method for single sign-on, which comprises the following steps: determining that a user requesting to log in a first application system does not log in other application systems; when the first application system is a first-level application system, providing user login information for a user-associated second-level authentication center for authentication by a first-level authentication center; after passing the authentication, generating a user identity token and providing the user identity token for the first application system by the first authentication center; when the first application system is second-level application, authenticating the user login information by the second-level authentication center to which the first application system belongs, and after passing the authentication, generating the user identity token which is provided for the first application system; and providing service pages for a user by the first application system according to the user identity token. The invention simultaneously discloses a system for the single sign-on. By adopting the method and the system, the hierarchical management can be performed on the authentication and the system pressure is dispersed effectively.