Abstract: Software defined network (SDN) is one of the most popular network technologies nowadays. SDN decouples the traditional control plane from the forwarding plane, resulting in many new security and management issues while performing centralized control. Meanwhile, the types of vulnerabilities are diverse in each layer and north-south trending interfaces of SDN, and the spread trend is quite different. Aiming at the effect of vulnerability propagation in/between layers of SDN as well as its suppression strategy, a formal model of vulnerability propagation for SDN based on Bio-PEPA is proposed in this paper. First of all, the basic syntax of Bio-PEPA is discussed, and its applicability to SDN with obvious hierarchical structure and the vulnerability propagation process with dynamic characteristic is illustrated. Then, the vulnerabilities existing in each layer of SDN are explored and modeled in terms of layers. Besides, by constructing a formal model for the process of vulnerability propagation in/between layers of SDN, the mechanism of vulnerability propagation is analyzed in two levels, horizontal (in layers) and vertical (between layers). In this way, the vulnerability propagation of SDN can be better suppressed. Finally, the simulation results show that the vulnerability propagation of SDN can be effectively retained by reducing the connection conversion rate, improving the detection conversion rate and repairing conversion rate. Our works provide a reference for the law of vulnerability propagation of SDN, so as to improve the security of SDN.