Based on a brief description of fuel tank washing inerting process, this paper builds a general mathematical model for multi-compartment fuel tank and provides the initial conditions for solution with Runge-Kutta method according to mass conservation, equation of Nitrogen-Oxygen dissolution and evolution and equation of gas flowing through orifice. With a four-compartment fuel tank as the research objective and adopting domestic separation membrane to generate Nitrogen enriched air (NEA),two patterns 9 the single-flow and dual-flow 9 are designed respectively. This paper performs washing inerting simulation on fuel tank and analyzes the variation of average Oxygen concentration in fuel tank and in each compartment with flight envelope. Research result indicates that both the two patterns can meet the requirements that the average Oxygen concentration of upper ullage in fuel tank should be lower than 12%. Although the single-flow pattern is simpler than the dual-flow, it cannot avoid external air entering into the fuel tank, which will cause the result that the Oxygen concentration in some compartments will be higher than 12% during diving phase while the dual-flow pattern can guarantee the Oxygen concentration of compartments not to exceed the limits during the whole flight.