Accurate heat flux measurement is essential for developing hypersonic vehicles and their thermal protection systems. To address the lack of reliable calibration methods for heat flux sensors under high-temperature, high-speed conditions, this study introduces a dual-plate transient calibration technique. A highly accurate thin-film platinum resistance sensor is used as a reference, installed alongside a Gardon gauge on a rapid-insertion mechanism within a wind tunnel. Calibration experiments conducted at 0.3 Ma and temperatures from 100 to 300 °C successfully determined the convective heat flux sensitivity coefficient of the Gardon gauge. This method provides a novel and effective approach for sensor calibration under extreme conditions, significantly improving measurement reliability and offering robust technical support for hypersonic vehicle development.