The metal chloride of KCl was chosen to modify Au-Cu/AC to decrease the noble metal of gold and enhance the catalytic performance. Then a mercury-free catalyst of Au-Cu-K/AC was prepared by the impregnation method. The catalytic performances of mercury-free catalyst for acetylene hydrochlorination were conducted for 1600h in a fixed bed reactor by a single-tube pilot unit. The fresh and used catalysts were also characterized in comparison. The results showed that the acetylene conversion on mercury-free catalyst decreased slowly from 98% to 89%, and the vinyl chloride monomer (VCM) selectivity was kept at 99.7% under reaction conditions of temperature 165 degrees C, gas hourly space velocity (GHSV) 40h(-1), and feed volume ratio of HCl to C2H2 of 1.05 during 1600h on stream. The results showed that the additives of K with Cu can make the active species of gold dispersed well and retard the aggregation of particles. The reason for the slow decline of acetylene conversion for Au-Cu-K/AC catalyst was the whisker carbon deposition, shown in a faint yellow colour over the catalyst surface, which consisted of short-chain hydrocarbon molecules. Further study for accelerated deactivation of sole metal in the catalyst gives the clues that the non-noble metal of Cu in Au-Cu-K/AC catalyst plays a key role to form the deposition.