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