Combustible gas within the limits of the explosive in sealed space of floating roof tank，could lead to explosion by lighting. In order to study the feasible technology about lighting of floating roof tank seal, concentration of oxygen remain below 8% was put forward through adding nitrogen into the sealed space, thus reduced the risk of explosion. By analyzing the change of oxygen concentration after injecting nitrogen into the sealed space, the relationship between the change of oxygen concentration and nitrogen injection was set up. At the same time, through the experiment when the sealed maintains the micro positive pressure, a formula was established about the nitrogen flow and micro positive pressure, which could obtain the amount of nitrogen directly under different wind speed and lightning time to complete inertly. The method provided a reference to blanketing technology applications in seals of floating roof tanks.