Activated carbon-supported mercuric chloride(HgCl_2) is used as an industrial catalyst for acetylene hydrochlorination. However, the characteristic of easy sublimation of HgCl_2 leads to the deactivation o the catalyst. Here, we showed that the thermal stability of the Hg/AC catalyst can be evidently improved when Cs Cl is added into the Hg/AC catalyst. Compared with the pure Hg/AC catalyst, the sublimation rate of HgCl_2 from the Hg–Cs/AC catalyst decreased significantly and the Hg–Cs/AC catalyst showed bette catalytic activity and stability in the reaction. This promoting effect is related to the existence of cesium mercuric chlorides(Cs_xHg_yCl_(x+2y)) highlighted by XRD, HR-TEM and EDX analyses. Thus, reacting HgCl_2 with alkali chlorides to form alkali-mercuric chlorides may be a key to design highly efficient and thermally stable mercuric chloride catalyst for hydrochlorination reactions. Activated carbon-supported mercuric chloride (HgCl_2) is used as an industrial catalyst for acetylene hydrochlorination. However, the characteristic of easy sublimation of HgCl_2 leads to the deactivation of the catalyst. Here, we showed that the thermal stability of the Hg / AC catalyst Compared with the pure Hg / AC catalyst, the sublimation rate of HgCl 2 from the Hg-Cs / AC catalyst decreased significantly and the Hg-Cs / AC catalyst showed bette catalytic activity and stability in the reaction. This promoting effect is related to the existence of cesium mercuric chlorides (Cs_xHg_yCl_ (x + 2y)) highlighted by XRD, HR-TEM and EDX analyzes. Thus, reacting HgCl_2 with alkali chlorides to form alkali- mercuric chlorides may be a key to design highly efficient and thermally stable mercuric chloride catalyst for hydrochlorination reactions.