To examine the plasma-assisted combustion of a scramjet, a microwave-enhanced gliding arc plasma method was proposed in this study, and the flame structure and combustion instability were observed. The mechanism of plasma-assisted combustion was obtained via a Bunsen experiment, and then the influence on supersonic combustion was obtained on a direct-connected scramjet. The active species of the flame was determined via optical emission spectroscopy, and the flame temperature was measured with a thermocouple. The luminous intensity of the OH radicals in the flame increased ninefold when the flame temperature was increased to 1573 K, but the luminous intensity of CH* and C-2 was not obviously changed with the excitation of arc plasma. Moreover, the DC arc plasma had no effect on the rotation and the vibration temperature of OH radicals under these experimental conditions. In the range of microwave energy less than 800 W, there was no typical change in the intensity of the radicals; however, when the microwave power was up to 1000 W, the effect became obvious. When plasma was applied to the scramjet, the plasma caused the pre-combustion shock train to move forward, and the initial and stable position of the flame was transferred from the cavity shear layer to the front of the fuel jet. These results clearly show that plasma free radical mechanisms cause changes to combustion modes.