Abstract: The experimental studies have been carried out to explore the influences of the Specific Heat Ratio (SHR) on the air-breathing hypersonic vehicle's nozzle plume at CARDC's 0.5m hypersonic wind tunnel. The SHR of the plume at the scramjet exit is simulated by CF₄+Air with SHR=1.25. The Laval nozzle is used in the experimental model to generate the supersonic nozzle plume and the flow field generated by the wind tunnel is used to simulate the model's external flow field. The differences of the pressure distributions and flow field structures are compared between the pure air and mixture gas plumes. Results show that in the mixture gas plume, the pressure distributions are higher than air plume in the core and interaction regions. The SHR is one of the key parameters for the study of air-breathing hypersonic vehicle's plume.