Abstract: On the 300 kW DC axial tube electrode arc heater, the U–I characteristics and thermal efficiency of CO₂ and air are measured by experiment, and the regression analysis is carried out by using the similarity criterion number. The unified relationship of electric and thermal characteristics that can be applied to the two media is obtained, and compared with similar heaters abroad. The results show that CO₂ and air arc heaters have similar electric and thermal characteristics, under the same input parameters (arc current and gas flow); the total pressure of CO₂ is 18% lower than that of air, but the arc voltage, enthalpy and thermal efficiency are 5.9%, 6.7% and 10.9% higher respectively; the regression errors of U–I characteristics and thermal efficiency are −13.0%～11.4% and −33.0%～34.7% respectively. This relationship plays an important guiding role in the operation and commissioning of the high-power arc heater.