The effect of quasi-DC discharge plasma filament on the cavity in a scramjet combustor is numerically investigated based on the dominant thermal mechanism. The electrodes are located near the upper edge of the back-step and at the bottom of the cavity. And plasma filament is formed in pulsed control mode. Change of prformace parameters such as wall pressure, shear layer, cavity drag force and mass ex-change rate under the different pulsed control frequencies is analyzed. Simulation results demonstrate that the first oscillation frequencies of the key points are close to the pulsed control frequencies, and other oscillation frequencies are the multiple of the pulsed control frequencies. The big recirculation zone is destroyed by the plasma filament, yet the cavity is preheated. The drag coefficient drops more apparently with higher control frequency, while the rate of mass exchange increases much more by the plasma, the mass exchange rate is higher under higher control frequency.