The invention relates to a method for improving the performance of a mercury catalyst when acetylene synthesizes vinyl chloride through hydrochlorination. The mercury catalyst takes activated charcoal as a carrier loaded with mercuric chloride or loaded with mercuric chloride and additives; phosphoric acid is added in collocated steeping fluid, wherein, the concentration of phosphoric acid ranges from 0.01 to 5 mol/L in steeping fluid; phosphoric acid and steeping fluid are stirred fully, and then activated charcoal is added, the amount of steeping fluid is 0.1 to 5.0 times of the saturated adsorption capacity of activated charcoal, and the steeping time ranges from 2 to 24 hours; and the catalyst is dried, and evaluation is needed. The catalyst improves the catalyst activity and greatly improves the stability of the catalyst at the same time during the process that acetylene synthesizes vinyl chloride through hydrochlorination, and achieves the functions of fully delivering the catalytic performance of mercury, fully utilizing mercury resources, inhibiting carbon deposition on the catalyst and the like; and in addition, the preparation method has the advantages of simplicity, low cost, good effect and excellent industrialized application prospect.