A Floating Production Storage and Offloading (FPSO) is used as a model to analyze risk consequences of oil and gas leakage and diffusion of topside module, to explore the causes and hazards rating of risk. According to the structural characteristics of the FPSO system, the fuzzy comprehensive evaluation method and the Graham (LEC) evaluation method are introduced to identify the risk of oil and gas leakage and diffusion for the topside module equipment of the FPSO, and the equipment that may cause risks is identified. Fluent software is used to simulate and analyze the consequences of gas diffusion in risky equipment under the conditions of constant and variable mass leakage rate, different wind direction and wind speed. The influence of gas diffusion on the consequences of gas leakage is summarized, and the measures on how to reduce the risk of oil and gas leakage of the topside module are put forward, which can be used as a reference for the arrangement and installation of the topside module equipment.