A weighted role for activating task and periodic time authorization is an unsolved major problem for the access control of a workflow management system (WfMS). In this paper, a novel weighted role and periodic time access control (WRPTAC) model for WfMS is proposed on the basis of a role-based access control model. The periodic time expression method is discussed and then the new authorization concepts and the temporal authorization derivation rules for WfMS are defined respectively. An algorithm based on the graph theory for verifying the consistency of all the authorization derivation rules is presented, which has the time complexity of O(n²). The constraint rule for activating task is defined, which can express complex access control constraints for WfMS.