The quasi-DC discharge plasma across the backward facing step of a flame-holding cavity can be used to improve the flame stabilization of the cavity in a scramjet combustor. The effects of plasma filaments on the main flow structures, cavity drag, mass exchange rate, cavity oscillating characteristics, stagnation pressure loss, and combustion efficiency are numerically investigated based on the dominant thermal blocking mechanism. The results show that the cavity shear layer fluctuates dramatically due to the 'cutting' effect of plasma, which leads to obvious changes in the fuel mixing and burning in the combustor, and the pressure distribution near the rear edge of the cavity. Some symmetrical structures in the shape of 'branch pipes' are observed, which represent the distribution of product water. The periodical producing of them can be attributed to the movement of the cavity shear layer. Compared with the no plasma case, the cavity drag coefficient overall increases 11.3%, yet the mass exchange rate rises sharply over 10 times. The sound pressure level of the cavity representative points is raised by the plasma, and the first dominant frequency of pressure oscillation at the monitor point on the rear edge is twice the plasma actuation frequency. Although the stagnation pressure loss increases, the plasma shows benefits in energy producing that outweigh the energy consumed by the plasma actuator.