©　2017,　Xi＇an　High　Voltage　Apparatus　Research　Institute　Co.,　Ltd.　All　right　reserved.　CO2　is　a　potential　alternative　for　SF6　in　the　high　voltage　circuit　breaker.　In　this　paper,　the　critical　re-duced　electric　field　strength　(E/N)cr　of　CO2,　CO2-O2,　CO2-N2,　CO2-CH4　and　CO2-H2　at　different　mixing　ratios,　pressures　and　temperatures　(300~4000　K)　have　been　calculated.　The　species　composition　of　CO2　gas　mixtures　was　calculated　based　on　the　Gibbs　free　energy　minimizing　to　pick　out　the　main　species.　Then　the　(E/N)cr　of　CO2　gas　mixtures　at　different　conditions　were　derived　by　solving　the　two　term　approximation　of　Boltzmann　equation.　The　re-sults　indicate　that　the　(E/N)cr　of　CO2-O2,　CO2-H2　and　CO2-CH4　are　higher　than　that　of　other　mixture　gases.　The　fun-damental　data　of　the　insulation　properties　of　CO2　and　its　mixtures　under　high　temperature　are　obtained,　which　pro-vides　a　theoretical　basis　for　the　search　of　substitute　gases　for　SF6.