CAS in Tomcat to achieve single sign-projects, SSO (Single Sign On, referred to as SSO) is currently the more popular services in the enterprise business as one integrated solution, SSO allows multiple applications in the system, users only need to sign in one will be able to access all of mutual trust applications. CAS (Central Authentication Service) is a good application for Web Single Sign-fr a mework, this paper introduces the principle of CAS, agreement