Based on the first principles ab initio method of density functional theory, the rutile TiO2(100) surface adsorption models were established. The mechanism of adding C to promote the chlorination process of TiO2 is revealed through the calculation and analysis of adsorption energy, charge density, state density. The simulated results indicated that the adsorption process of Cl2 on TiO2(100) surface was physical adsorption. The adsorption process of both C and Cl2 on TiO2(100) surface was chemical adsorption. The addition of C atom promoted the dissociation of Cl2, and the formation of C?Cl bond or Ti(5c)?Cl bond. Moreover, the stability of the system containing C?O(2c) bond was higher than that of the system containing C?O(3c) bond. The C atom can bond with O(2c) or O(3c), and weaken the strength of Ti(5c)?O(2c) bond. Thus, the dissociated Cl atom tended to bond with Ti(5c) on the surface of TiO2(100) or with C atom. It was found that the number of Ti(5c)-Cl bonds in the system had certain influence on the stability of the system. When one of the dissociated Cl atoms in the system was free, the other Cl atom bonded with C atom, and the adsorption energy of the system was the highest for ?5.25 eV, its stability was lowest. When the two dissociated Cl atoms form Ti(5c)?Cl bond and C?Cl bond with Ti(5c) respectively. The adsorption energy of the system was the second for ?6.75 eV. When two Cl atoms and surface Ti(5c) form Ti(5c)?Cl bond, the adsorption energy of the system was the lowest for ?7.78 eV and the structure was the most stable. At this point, the bond between C and O(2c) was the strongest in this structural system, and more electrons were transferred around the Cl atoms. It indicated that the addition of C can form the C?O(2c) double bond, promote the dissociation of Cl2, and adsorption reaction on the surface of TiO2 (100) which played an important role in improving the chlorination process of TiO2.