The pressure oscillation causing combustion instability has been a persistent challenge for researchers and designers to design a stable solid rocket propulsion system. The acoustic vortex coupling in the combustion chamber caused by vortex shedding is one of several mechanisms that drive pressure oscillation in motors.Taking the Clx motor with a typical backward-facing step configuration, in which acoustic vortex coupling occurs, as a reference, the plasma introduced into the Clx motor is expected to control the vortex movement and hence disrupt the frequency coupling between the vortex and the acoustic. A comprehensive three-dimensional large eddy simulation is conducted to analyze the motion patterns of vortices and the pressure oscillation characteristics within the combustion chamber with and without plasma. The primary objective is to clear the influence of plasma on the acoustic vortex coupling and pressure oscillation of the Clx motor. The numerical computation results reveal that: compared with the experimental data, the maximum error of the calculation on the pressure oscillation frequency is 6.52%. This means that the large eddy numerical methodology established in this paper can effectively work; the incorporation of plasma causes a pronounced advancement in the turbulent flow occurence position behind the step. Meanwhile, the energy of vortex structures progressing towards to the nozzle inlet is reduced; both the vortex intensity and the acoustic feedback at the nozzle inlet are reduced by the plasma actuation. All of those phenomena effectively mitigate the pressure oscillation caused by acoustic vortex coupling.