In order to ensure the security of the sharing service in grid, a dynamic authorization model based on trust mechanism is presented, which is based on role-based access control. In the model authorization is obtained while the trust worthiness of the bargainers exceeds the trade threshold; it also introduces the concepts of task and condition into the authorization model so that it can achieve dynamic authorization based on extended RBAC. Experimental analysis shows that the model can effectively enhance the access control efficiency of the sharing service between grid nodes.