In this paper, ReaxFF simulation is used to study the friction characteristics of amorphous carbon (a-C) films in CO2 environment by considering the influences of CO2 molecular number and the normal load. The results manifest that the decrease of friction at the beginning of sliding stage is mainly due to the physical adsorption of CO2 molecules, while in the stable stage, the low and stable friction is caused by the combination of chemical adsorption and physical adsorption. The large amount of CO2 molecules greatly reduce the friction force by generating more C-O terminal groups, which are not easily damaged by the increased normal load. In addition, due to the presence of CO2 molecules, graphitization has little effect on the friction behaviors of a-C films in CO2 environment.