The invention discloses a WEB single sign-on method completely transparent to a user and an application. The method comprises the following steps that: 1, an HTTP (Hyper Text Transport Protocol) data packet between the user (a browser) and the WEB application is intercepted by a transparent bridge technology, 2, the user is redirected to an SSO (Single Sign-on) server by a page redirection technology during first access, 3, after SSO service is certified, the browser is redirected back to the WEB application and a security Token is set, and 4, when the browser accesses the application again, an SSO gateway inspects the security Token, and the access is passed if the security Token is valid. According to the WEB single sign-on method, the single sign-on of the WEB application can be achieved under the conditions that a client or plugin is not required to be installed and the application is not required to be modified, and a top-level domain can be crossed.