Flexible task distribution is one of critical enabling technology for adaptive workflow management system (WFMS). Most of workflow systems support role based task distribution, which is driven by authorizations. However, quantitative aspects, such as proficiency, workload and task urgency, are not taken into account. In this paper, based on the role based task distribution model, a fuzzy synthesis evaluation hierarchy is proposed from three aspects including task, role and actor. The indexes of the hierarchy are related to the status or history of the attributes of these three aspects. When a task is offered to too many or too few workers sharing the required role, this hierarchy can be evaluated and used to select the more appropriate worker or add less-qualified workers to improve the performance of workflow.