Multi-instance activity can express the scenario, in which many people conduct the same task. Supporting multiple instances of one activity can enable workflow management system to be flexible on handling workflow process. But some problems emerge, such as synchronizing schedule of multi-instance and conflict of data share. The paper proposes a scheme of “Group-Categorization”, which solves the synchronization among multi-instance in one activity, when workflow is executed. It starts activity multi-instance with groups, which is synchronized by categorization in one group, so that there is no need to extend the standard workflow model. Furthermore, it has the capability of workflow language expression. Pointer strategy solves the conflict, when activity multi-instance access same data. According to the unique of process instances AID, it can share read-only data and can copy writing data without extra data attributes. It is available for standard workflow model.