A rapidly changing environment forces the workflow management systems to change their workflow processes ever more frequently. In order to improve the flexibility of workflow management systems, a dynamic workflow model is proposed in this study. The concepts of flexible activities and historical execution information are put forward to construct dynamic workflow models in this method. Each flexible activity is used to encapsulate a group of indeterminate factors, e.g., the constraint rules and optional sub-activities when reifying a flexible activity. Historical execution information is the executive logging of a previous workflow instance. Two algorithms are put forward to guarantee the correctness of sub-workflows and the global control of dynamic processes. Furthermore, a simple example is given to validate the proposed dynamic workflow model. However, this method cannot perform well if there existing loop structures in sub-workflows and the reifying processes of flexible activities are not intelligent enough.