Access control is an important mechanism for enhancing workflow system security．Role-based access contro1 model(RBAC)is used in the most of workflow systems, and it has become a research topic in the area of workflow．Based on the traditional RBAC model and embarked from decoupling the workflow access control model from the process model，a T-RBAC was presented, which based on identity authentication and role authorization．Workflow tasks were run dynamically by using T-RBAC. The model to describe dynamic workflow system can enhance workflow system security and provide flexibility in access control system. And the model is implemented in a company project.