In order to study the combustion characteristics of n-heptane under O2/CO2 atmosphere, a C-H combustion mechanism based on the detailed reaction paths of CO2 and H. was proposed. The possible reaction paths of CO2 and H. was analyzed by using density functional theory, and the calculation grids were established according to the actual size of constant volume combustion chamber. The combustion process of n-heptane under different atmospheres (air, 53%O2/47%CO2, 61%O2/39%CO2) was calculated through C-H mechanism. A visual experimental platform of constant volume combustion chamber was built to observe the combustion process of n-heptane under different atmospheres. The reaction sites of CO2, the reaction energy barrier of CO2 + H.→ CO +.OH and the flame length were analyzed. The results show that the C-H mechanism can well predict the combustion flame length of n-heptane under O2/CO2 atmosphere, and the maximum errors are 6.33% and 10.71% respectively under 53%O2/47%CO2 and 61%O2/39%CO2.The reactivity of oxygen atom of CO2 is higher than that of carbon atom, and the average local ionization energy and molecular surface electrostatic potential at the oxygen end are 297.72 and -13.08 kcal/mol, respectively. H. can combine with carbon atom and oxygen atom of CO2, the reaction energy barrier are 26.71 and 11.07 kcal/mol, respectively.