Abstract: Studying the partial discharge characteristics of C₄F₇N/CO₂/O₂ environmentally friendly insulating gas can provide technical supports for the design, manufacture, and operation and maintenance of environmentally friendly gas-insulated electrical equipment. The partial discharge initial voltage (PDIV) of C₄F₇N/CO₂/O₂ mixture gas in an extremely non-uniform electric field is studied by using a needle-plate electrode, and the effects of O₂ volume fraction, air pressure and electric field non-uniformity coefficient on PDIV are analyzed. The experimental results show that, in the pressure range of 0.15~0.25 MPa, the volume fraction of O₂ added into the C₄F₇N/CO₂ mixture should not exceed 4%, while in the pressure range of 0.35~0.65 MPa, among the three proportions of O₂, the scheme of adding 4% O₂ by volume to the C₄F₇N/CO₂ mixture is relatively optimal. The "hump effect" will appear in PDIV of mixed gas with the change of air pressure. When the volume fraction of O₂ is less than 8%, the increase of the inhomogeneity coefficient can effectively reduce the difference of PDIV amplitude between C₄F₇N/CO₂/O₂ and C₄F₇N/CO₂, however, when the volume fraction of O₂ is higher than 8%, the change of inhomogeneity coefficient has a negligible influence on the change of amplitude difference between the two mixed gases. Therefore, from the perspective of the initial voltage of the mixed gas AC partial discharge, the C₄F₇N/CO₂/O₂ mixed gas has the feasibility of being applied to gas-insulated AC equipment.