The carbon flow model was established based on Substance Flow Analysis (SFA) and the factors that cause the carbon emission were analysis by mathematical programming. The evaluation indicators were defined in order to evaluate the carbon flowing of TDCS. The results show that the main form of carbon emission is the carbon gases discharged to the environment for TDCS. The main sub-process of carbon emission is the preheating subprocess. And the carbon emission increases with the increasing of fuel consumption and C/H ratio, which influence the carbon emission most.