The sealing structure between floating plate and tank wall in an oil storage tank with inner floating roof is critical to reducing oil vapor loss of the tank.A large oil storage tank with internal floating roof was selected and studied.Based on RNG k-ε theory,governing equations of oil vapor leakage in the floating roof tank were established,and a twodimensional model for oil vapor leakage evaluation in the floating roof tank was built using the Gambit software.The CFD numerical simulation method was used to optimize the labyrinth seal structure of the tank.The effects of the labyrinth seal tooth height,the tooth width and the tooth gap on the oil vapor loss were studied.The simulated results show that the oil vapor leakage is initially decreased and then increased with the increase of the tooth height,tooth width and tooth gap.When the tooth height is 2 mm,the tooth width is 2.9 mm,and the tooth gap is 0.6 mm,there are low oil vapor leakage,and good performance of the labyrinth seal in the tank.