Abstract: Surface charge accumulation characteristics of SF₆ have been extensively studied; however, little attention as has been paid to the C₄F₇N/CO₂ mixture which is the most concerned eco-friendly insulation gas. In order to clarify the surface charge accumulation characteristics in C₄F₇N/CO₂ mixture, downsized spacer samples with coaxial cylindrical electrode structure were adopted, and HVDC was applied on. The surface charge distribution in C₄F₇N/CO₂ mixture under pressures of 0.1MPa, 0.2MPa and 0.3MPa, and molar contents of 5%, 10%, 15% and 20% was measured. The results show that the surface charge morphology presents 3 modes in C₄F₇N/CO₂ mixture: irregularly shaped homo-charges injected area near the HV electrode, hetero-charges spot near the HV electrode, and bipolar cloud-like charges on the spacer surface. As the pressure increases, the hetero-charge spot expands, and the injected area and the cloud-like charges are reduced. As the content increases, the injected area charge density, the hetero-charges spot and the cloud-like charges increase. Compared to SF₆, C₄F₇N/CO₂ mixture is more sensitive to the electric field, enhancing gas ionization and gas conduction. The hetero-charges generated in the gas side are deposited on the surface. More charges are accumulated in C₄F₇N/CO₂ mixture, and necessary measures should be taken to suppress the surface charge for practical application.