A multi-channel gliding arc (MCGA) plasma is utilized to assist the lean blow-off flame in the scramjet combustor. Optical measurements including high-speed flame chemiluminescence, high-speed CH*emissions, and high-speed schlieren are used to characterize the reaction zone coupled with the flow field assisted by the plasma. The results show that the lean blow-off limit of the cavity shear-layer flame is extended by 21% when the MCGA plasma is added. The flame is blown out eventually due to a small amount of heat release near the lean blow-off limit. While the plasma is applied, the plasma can sustain the flame in the whole combustion process. The center of the flame tends to be concentrated near the plasma region, indicating that the flame is more likely to attach to the MCGA plasma. A plausible mechanism for the extension of the lean blow-off limit is revealed that the temperature is enhanced by the heat and active radicals produced by the MCGA plasma, which sustain the flame near the plasma region. Subsequently, the small flakes of the flame spread to the half back of the cavity to form the global flame with approximate local conditions.