Vulnerability is the essential reason of security and dependability, which has become a serious problemforWireless Sensor Network (WSN) while a lot of additional vulnerabilities are brought out by interactions among nodes and even more than inherent ones. Avulnerability diffusion model for WSN is proposed in this paper. Compared to current vulnerability diffusion model, we take the characteristic of heterogeneous of vulnerability diffusion into consideration, that is, the vulnerability diffusion between clusters is not the same as in a single cluster. Inspired by epidemiologicalmodel, we use the characteristics of static hierarchy of Bio-PEPA, a process algebra for the analysis of biological systems, to describe vulnerability diffusion in different scenes, including vulnerability diffusion in single cluster, vulnerability diffusion in multi-cluster without migration or with migration. The experimental results show that the vulnerability diffusion can be reduced by enhancing the recovery capability and decreasing the rate of vulnerability migration between clusters. Our works provide an insight into the nature of vulnerability propagation, and is useful to improve the security of WSN.