To meet the authorization administration requirements in a distributed computer network environment, this paper extends the role-based access control model with multiple application dimensions and establishes a new access control model ED-RBAC(Extended Role Based Access Control Model) for the distributed environment. We propose an extendable hierarchical authorization assignment framework and design effective role-registering, role-applying and role-assigning protocol with symmetric and asymmetric cryptographic systems. The model can be used to simplify authorization administration in a distributed environment with multiple applications.