This paper addresses a Pet ri—net based approach to supporting specification construction and execution of software system．On the basis of requirement descriptive model，a kind of specification represented in Petri一net is formed，which can generate the functional model on the implementation level by applying transformation and refinement．The specification is validated and verified by different simulation mechanism．This paper has put forward a kind of specificeation framework and may provide a novel idea for soft-ware automatic generation from the specification Ievel．