The catalytic mechanism of Ru-based catalysts in the acetylene hydrochlorination reaction has been investigated via the density functional theory (DFT) method. To study the effect of the chlorine coordination number on the catalytic mechanism, Ru₃Cl₉, Ru₃Cl₇, Ru₅Cl₇, Ru₃Cl₃ and Ru₃ clusters were chosen as the catalytic models. Our results show that the energy barrier for acetylene hydrochlorination on Ru₃Cl₉ was as high as 1.51 eV at 458 K. When the chlorine coordination number decreased, the energy barriers on Ru₃Cl₇, Ru₅Cl₇, Ru₃Cl₃ and Ru₃ were 1.29, 0.89, 1.01 and 1.42 eV, respectively. On Ru₃Cl₉, the H and Cl atoms of HCl were simultaneously added to C₂H₂ to form C₂H₃Cl, while the reaction was divided into two steps on Ru₃Cl₇, Ru₃Cl₃ and Ru₃ clusters. The first step was the addition of H atom of HCl to C₂H₂ to form C₂H₃˙, and the second step was the addition of Cl atom to C₂H₃˙ to form C₂H₃Cl. The step involving the addition of Cl was the rate-controlling step during the whole reaction. On Ru₅Cl₇ cluster, there was an additional step before the steps involving the addition of H and Cl: the transfer of H atom from HCl to Ru atom. This step was the rate-controlling step during the reaction of acetylene hydrochlorination on Ru₅Cl₇ and its energy barrier was the lowest among all the above-mentioned catalytic models. Therefore, the Ru₅Cl₇ cluster played the most predominant role in acetylene hydrochlorination with the largest reaction rate constant k_TST of 10³.