Abstract: The inerting for the oil vapor space of floating roof tank is a new method of fireproofing and anti-explosion. According to the principle of nitrogen inerting for the oil vapor space, a research on the inerting of nitrogen to oil vapor space of large-scale double sealing floating roof tank was carried out. In the research, the geometry and mathematical models of gas flowrate were established. PHOENICS software was used in the numerical simulations. The volume fraction distribution of nitrogen injecting oil vapor space at different rates was obtained under the condition of the nitrogen flowrate at 27 Nm³/h. The results showed that nitrogen flowrate had little effect on the distribution of volume fraction of oil vapor space. Volume fraction for oil vapor at the annular cylindrical space of primary and second sealing of floating roof tank achieved to the safe standards when the nitrogen was continuously pumped up to 230 minutes. The authors consider that the research conclusion has a guiding significance in fireproofing and anti-explosion of the floating roof tank with nitrogen inerting in project.