Mercuric chloride supported on activated carbon(HgCl_2/AC) is used as an industrial catalyst for the hydrochlorination of acetylene. Loss of HgCl_2 by sublimating from the surface of activated carbon causes the irreversible deactivation of mercury catalyst and environmental pollution. In this work, a ligand coordination approach based on the Principle of Hard and Soft Acids and Bases(HSAB) was employed to design more stable lowmercury catalyst. The low-mercury catalysts(4% HgCl_2 loading) were prepared by using HgCl_2 and potassium halides(KX, X = Cl, I) as precursors. The HgCl_2-4KI/AC catalyst showed best catalytic stability than HgCl_2/AC and HgCl_2-4KCl/AC in the hydrochloriantion of acetylene. HgCl_2 could form more stable complex with KI,K_2HgI_4 as the main active component of the HgCl_2-4KI/AC catalyst. The characterizations of XRD and EDX analysis illustrated that the active component of HgCl_2-4KI/AC was highly dispersed on the surface of activated carbon.The sublimation rates of HgCl_2 from the catalysts verified that the active component with larger stability constant had better thermal stability. Using Hg(Ⅱ) complexes with high stability constant as the active component may be the research direction of developing highly stable low-mercury catalyst for the hydrochlorination of acetylene. Mercuric chloride supported on activated carbon (HgCl 2 / AC) is used as an industrial catalyst for the hydrochlorination of acetylene. Loss of HgCl_2 by sublimating from the surface of activated carbon causes the irreversible deactivation of mercury catalyst and environmental pollution. In this work, a ligand coordination approach based on the Principle of Hard and Soft Acids and Bases (HSAB) was employed to design more stable low mercury catalyst. The low-mercury catalysts (4% HgCl_2 loading) were prepared by using HgCl_2 and potassium halides (KX, X = Cl, I) as precursors. The HgCl_2-4KI / AC catalyst showed best catalytic activity than HgCl_2 / AC and HgCl_2-4KCl / AC in the hydrochloriantion of acetylene. HgCl_2 could form more stable complex with KI, K_2HgI_4 as the main active component of the HgCl_2-4KI / AC catalyst. The characterizations of XRD and EDX analysis illustrated that the active component of HgCl_2-4KI / AC was highly dispersed on the surface of activated carbon. The sublimation rate s of HgCl 2 from the catalysts verified that the active component with higher stability constant had had better thermal stability. Using Hg (Ⅱ) complexes with high stability constant as the active component may be the research direction of developing highly stable low-mercury catalyst for the hydrochlorination of acetylene.