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Gas exchange instability is an important issue that affects the stability of the linear engine generator (LEG). Fuel injection quantity fluctuation is a potential disturbance factor that induces LEG gas exchange instability. In order to grasp the impact of fuel injection quantity changes on LEG under the action of multi-field coupling the effect of gas exchange stability, a numerical model of the coupling of motion-combustion-gas exchange multiprocessing is constructed, and the calculation method of cyclic iteration of the simulation results of each process is adopted to simulate and study the performance characteristics of the LEG gas exchange process under the fluctuation of fuel injection. The results demonstrate that the reciprocating frequency, movement stroke of LEG, and the fluctuation of fuel injection quantity present positive correlation characteristics. The increase of fuel injection quantity causes the piston to move faster near the reciprocating dead point, so the time provided for the gas exchange flow is compressed. Moreover, the increase in fuel injection quantity means that more fuel participates in the combustion process, releases more heat, and enhances the pressure and temperature of the gas in the cylinder. Although this is conducive to the rapid discharge of more combustion exhaust gas in the free exhaust phase, due to the shortening of the gas exchange flow time. The actual exhaust gas scavenging efficiency decreases with the increase of the fuel injection quantity and the corresponding fresh charge trapping efficiency shows an increasing trend.