A large volume of waste mercury chloride catalyst (HgCl 2 ) is generated by calcium carbide-based polyvinyl chloride production in China, which accounts for 30% of global mercury consumption. However, there are few studies on the leaching behavior and risk control of waste catalyst where mercury is in the form of HgCl 2 , which is more soluble than metal mercury and poses great threat to human health and environment. This study investigated mechanochemical stabilization and leaching behavior of waste mercury catalyst using sulfur as additive. We found that mercuric sulfide (HgS) formed through mechanical milling, although the reaction between HgCl 2 and S is not thermodynamically favored. Moreover, toxicity characteristic leaching procedure (TCLP) results showed mercury leaching values are lower than the TCLP limit value of 0.2 mg/L after mechanochemical treatment. Results of sequential extraction procedure showed that ball mill could transfer more than 99.9% of the Hg in the catalyst to the least mobile fraction.