To investigate the mechanism of the heterogenous reactions during the Chemical vapor infiltration (CVI) process of carbon/carbon composites, the dissociation of CH4 on graphene was calculated by density functional theory (DFT). Graphene was used as the adsorption surface in the course of the heterogenous reactions. Based on the energy analysis, the preferred adsorption sites of CHx(x = 0-4) and H on graphene were obtained. Then, the stable co-adsorption configurations of CHx/H(x = 0-3) on graphene were located. The calculation results show that CH4, CH3 and H prefer to be adsorbed at the top of a carbon atom of graphene, while CH2, CH and C are favorable on the midpoint of a C-C bond of graphene. Transition state (TS) calculation shows that the dissociation of CH(4)into CH3 and H is a rate-determining step. Additionally, by comparing the dissociation of CH3 into CH2 and H and the formation of C2H6 during the dissociation of CH4, it is obvious that the CH3 groups are more likely to produce ethane rather than dissociating into CH2 and H.