Aiming at the main defect of typically dynamic workflow systems that is not able to decide automatically the dependences among data in workflows, the Petri-Net is used to model data dependence for the dynamic workflows, the data structure of dynamic workflow denoted by Petri-Net is described in detail. Based on the model and data structure a decision algorithm of data dependence of workflow is proposed, and the complexity of the algorithm is analyzed. The basic idea of the algorithm is to suppose if a workflow can be dealt with normally ( i.e. conform to data dependence ), then there must be a state in the reachable states of DataFlow in which a mark will appear in the postposition place of data transition. Lastly, by a computing example the decision algorithm of dynamic workflow based on Petri-Net is proved to be practical, effective and feasible.