Based on low temperature controlled oxygen consumption catalytic inerting technology, a mathematic model of the equilibrium oxygen concentration of the fuel tank ullage with mixed inert gas is established using the method of differential element calculation. The relationship between equilibrium oxygen concentration and height is calculated by the model with the consideration of fuel temperature, fuel load and gas component and compared with fuel tank using nitrogen enriched air. The results show that when mixed inert gas is used to inert the fuel tank on the ground, the equilibrium oxygen concentration decreases with the increase of flight height, while it is completely the opposite when the nitrogen enriched air is used. The higher the fuel load is, the greater the variation range of oxygen concentration in the ullage. The increase in fuel temperature is bad for fuel tank with mixed inert gas, but good for those with nitrogen enriched air. In general, the use of mixed inert gas to inert fuel tank can effectively suppress the adverse trend of the fuel tank flammability increasing with flight height, and the study results can provide theoretical basis for ground inerting and fuel pre-scrubbing technology using mixed inert gas.