Strict Integrity Policy is one of important security policies. In this paper, based on Petri net and Biba Model, the Petri net-based definitions of Strict Integrity Policy are formally described in detail. The Petri net-based definitions and the coverability graph allow one to analyze and verify Strict Integrity Policy in Petri net model of a system. Subsequently, an example of the integrity policy is illustrated and the conclusions show that Petri net is not only a concise graphic analysis method, but also suited to formal verification. This verification approach can efficiently improve the integrity policies during the system security design and implementation.