Security assessment with vulnerability is based on American Petroleum Institute (API) Standard 780 Security Risk Assessment (SRA) Methodology which gives fixed steps combining with vulnerability, As Low As Reasonably Practicable, Swiss Cheese Model and cost-effectiveness theory. Vulnerability of different periods were analyzed emphatically, such as vulnerability before accidents, vulnerability in accidents and vulnerability after accidents. As Low As Reasonably Practicable can allow managers to know clearly that which assets should be protected deeply. Swiss Cheese Model helps analyzers understand detailed reasons of attacks and avoid adversaries’ attacks. Through cost-effectiveness theory, benefits of the security investments can be maximized. Overall, this article can make managers know more about the risk level in multiple dimensions and enlighten analyzers to assess the risk level of chemical plants.