Aiming at the complex demand of grid access control, the current task-role -based access control model for grid was improved and a new model based on role-task role-based access control (G-R_TRBAC) was built. In G-R_TRBAC model, Role-Based Access Control (RBAC) combined with certification management system was applied in the virtual organizational domains of the grid environment. The virtual organizational domain of the grid environment was an improved model from the task-role-based access control model, which could meet the requirements of multi-domain and dynamic characteristics in the grid access control.