Abstract: SF6 with strong greenhouse effect is still widely used as insulation medium for DC wall bushing, and C4F7N/CO2 mixed gas has attracted attention as a potential SF6 substitute gas. In this paper, the liquefaction temperature of the C4F7N/CO2 mixed gas is first fitted, and the dielectric strength of the 4%-6%C4F7N/CO2 mixed gas is calculated. When the air pressure is 0.6MPa, the breakdown field strength is about 18.03kV/mm under power frequency voltage and about 21.56kV/mm under lightning impulse. Secondly, a 3-D model of the DC wall bushing is established, and the internal electric field distribution is simulated and calculated. The maximum field strength is 25.24kV/mm at the end of the shield. Finally, the shield structure is optimized by BP neural network, the maximum value of the field strength is reduced by about 7.9kV/mm and the electric field nonuniformity is reduced to 3.2 after optimization. Based on the breakdown field strength, liquefaction temperature and other factors, the 4%-6% C4F7N/CO2 gas mixture at 0.6MPa has the potential to be applied to ± 400kV DC wall bushing.