Abstract: For tackling the deficiencies of weak adaptability due to the singleness of the role establishment method, role or privilege redundancy, and little attention on resource management in the existing Role-Based Access Control (RBAC) researches, a Scalable Access Control model Based on Double-Tier Role and Organization (SDTR-OBAC) is proposed. Through double role partition, a double-tier role architecture of function role and task role is presented, solving the problem that the traditional role can not cover the requirements of both organizational level and application level at the same time. The concept of organization is introduced to integrate with the double-tier role and form an organization-role pair assigned to user instead of role only in RBAC, making model suitable to cross-domain access as well as a single domain. Through extending privileges as an operation and resource type pair, the model and its constraints including separation of duty and cardinality constraint are defined formally. The discussion of expressive power and complexity indicates that SDTR-OBAC retains all the advantages of RBAC, and can effectively reduce the administration complexity with better scalability and universality.