For public service platform of science and technology evaluation, a cluster Central Authentication System (CAS) model with intrusion tolerance is introduced to Single Sign On (SSO) system. It aims at improving the single point of failure and security problem which result from using original CAS. This paper first presents the proposed single sign on system structure and cluster CAS model. Then it elaborates design and implementation of SSO system, which include the unified authentication service module, authentication server controller module, register module, user information module and unified user backstage administration module. Using the single sign on system based on cluster CAS, users can access all trusted applications when they login in the platform only once. Besides, the cluster CAS model reduces the complexity of user management, improves single point of failure and enhances security of authentication server.