Sulfur hexafluoride (SF_6),as the commonly used insulating gas,has high global warming potential. Iodotri-fluoromethane (CF_3I) is a potential alternative to SF_6,which has the relatively high price and boiling point. So the gas mixtures with low proportion CF_3I may avoid these problems and have better application prospects. We studied the breakdown voltages of CF_3I-CO_2 gas mixtures with low proportion of CF_3I in extreme non-uniform electric field. The tests covered different mixing ratios, different gas pressures, and different gap distances. The results show that the minute quality of CF_3I can significantly increase the breakdown voltage and become stabilized while the proportion of CF_3I is more than 6%. Though it is still lower than SF_6, the gases mixtures of CF_3I and CO_2 have shown a good positive synergistic effect with minute quality of CF_3I, thus the low proportion CF_3I gas mixtures have application feasibilities and cost advantages in large gap distance system.