Abstract: The model formally describes the key elements of access control such as role,user,privilege,task unit,authorization strategy,authorization constraint and the relationship between these elements.We identify the following four types of authorization constraints：require role constraints,require user constraints,deny role constraints and deny user constraints,then provide constraint consistency checking rules upon them.In this model,authorization flow is synchronized with workflow and the workflow can be efficiently executed through the constraint consistency checking rules.The main advantage of the model is its comprehensiveness,flexibility and practicability.