Abstract: C₄F₇N/CO₂ mixed gas, as a new type of environment-friendly insulating gas, has been applied in practical equipment. Timely detection to ensure that the mixing proportion of mixed gas remains stable is the key to ensure the insulation reliability of equipment. In this paper, a new molar ratio detection method of C₄F₇N gas in mixed gas is proposed. In this method, the voltage value corresponding to townsend discharge current under certain conditions is adopted to characterize the C₄F₇N gas mixing ratio. Firstly, the steady-state townsend discharge test of C₄F₇N/CO₂ mixed gas under different mixing ratios and different pressures was carried out, and the relationship between molar ratio and voltage was established. It is found that the proposed method can be adopted to detect the mixing ratio range of 0~100%, and the detection error in the commonly-used range of 5%~30% is less than 0.4%. Then, the feasibility of the proposed method is theoretically analyzed, and it is proved that there is a linear relationship between the voltage value corresponding to the townsend discharge current and the mixing ratio when the mixing ratio of C₄F₇N/CO₂ is in the range of 0~100%. Finally, the method proposed in this paper was compared with chromatography and infrared/ultraviolet spectroscopy, and the unique advantages of the detection method in cost and gas consumption are pointed out. The results can provide theoretical and experimental bases for the practical application of this method.