Verification of workflow soundness is a problem that has not been solved well in the workflow research. Many workflow management systems lack of the effective tolls which can guarantee the correctness of workflow. In this paper, a necessary condition of sound workflow based on the event balance is put forward. Theorem of event balance in the workflow execution history is proved. Also the calculation of event balance is introduced in the workflow model in order to analyze the model structure which may destroy the soundness of workf;ow.This method is not bouud to any specific workflow model so that it can be used in a wide range of models.Also this method can be finished in polynomial time.