Abstract: In light of the adverse effect of the nitrogen overflow from the secondary seal of the large oil storage tanks protection system during the active protection process, a nitrogen diffusion model is established, and the diffusion source strength, the diffusion coefficient and the diffusion height are analyzed and determined. The nitrogen spillover diffusion are simulated and analyzed by using Matlabd software, and the safety status of nitrogen spillover is evaluated. It can be concluded that there is no injury area and lethal area in the process of nitrogen in the active protection process, and there is almost no impact on human body. Meanwhile, a model was used to test the nitrogen spillover in the most unfavorable diffusion state. The results show that the leakage intensity of nitrogen is far less than that of simulation analysis, and the safety of human body near the leakage source will not be affected. Finally, through the test of the actual oil tank, the test of gas spillover on the actual oil thank is expounded, and the advancement, practicability and safety of the active protection system for the large oil storage tanks are evaluated.