Petrochemical enterprises have made an indelible contribution to the global modernization. However, as the high risk-level enterprises, once gas leak occurs, it will cause great harm to the surrounding environment and residents. At present, there still exists some weakness in China’s research on petrochemical hazardous gas leak detection and risk classification is still weak. Based on odour recognition (rotten egg odour) and CFD simulation technology, this paper conducts analysis of risk levels for the hydrogenation process in petrochemical processing. The study has found that CFD can be used to simulate the concentration of harmful gas (H2S) diffusion and characterize the severity of harmful gas leakage; the wind direction of the hydrogenation unit and the installation and arrangement of the equipment have an important impact on the leakage of harmful gases, and then CFD simulation can be used to analyse the risk level. This is of guiding significance for the placement of H2S detector.