When hypersonic vehicles are flying fast, the vehicle and atmosphere are heated up by intense friction, resulting in the dissociation or ionisation of the surface atmosphere and the formation of a plasma sheath. Plasma sheath interferes with electromagnetic wave transmissions and can cause severe interruptions to vehicle communication signals, forming a “blackout”, which poses a great threat to the safety of hypersonic vehicles. Due to its high peak power, high power microwave will change its electromagnetic characteristics after irradiating the plasma sheath, and scholars have already carried out the research of mitigating the problem of blackout of the vehicle through high power microwave irradiation. However, the flight state of hypersonic vehicles will seriously affect the electromagnetic properties of the plasma sheath, which in turn affects the irradiation effect of high-power microwaves. In this paper, plasma sheaths were irradiated for a certain period of time at different flight altitudes and flight speeds using high-power microwaves, and an increase in low-frequency transmission of electromagnetic waves has been found at low altitudes and high speeds, which is helpful for the alleviation of the blackout. It is essential for alleviating the blackout of hypersonic vehicles.