Cloud computing is a large-scale distributed computing mode, which forms an abstract, virtual, and dynamically scalable resource pool. The resource pool can provide services including computing, storage, development platform and software to users through Internet on demand. Therefore, it possesses powerful computing and storage capacity and even lots of other advantages. In this paper, the petrochemical plant safety management system (PSMS) proposed based on cloud computing whose function can be extended dynamically. It includes the online database concerning process safety knowledge, production-related information system which can be used to train operators and fulfill online inquiry. It also contains hazard fingerprints libraries and the hazard countermeasure libraries of chemical process. PSMS can executive computations with different scales and types based on massive real-time data collected from process. And the computation results can be matched with the hazard fingerprints database to evaluate and predict the process safety situation. If PSMS interacts with the DCS, the process may eliminate hazard by itself based on the hazard countermeasure database. Therefore, the real time information and safety knowledge of process can be organized to form wisdom, which would enhance the process safety greatly. The computing power of the cloud computing can guarantee the model computation completed just in time. The powerful sever cluster and highly reliable data center of the cloud computing guarantee that the information system, hazard fingerprints database and hazard countermeasure database can be extended dynamically, which also guarantee the in-time matching among computation results, hazard fingerprints, hazard fingerprints and its countermeasures. In order to take full advantages of cloud computing to make it easy to expand and upgrade, and then to solve the problem that the different petrochemical production system needs different services, the software will be developed with service oriented architecture (SOA) which is more abstract than object oriented architecture (OOA).