A systematic theoretical study on the effect of H2S on mercury (Hg) adsorption on activated carbon was conducted by density functional calculation. The form and amount of sulfur species formed by H2S adsorption dominate the pattern of Hg adsorption on activated carbon surface. C-S species is more active to improve Hg adsorption than C-S-C species. C-S species can improve the chemisorption of Hg by enhancing the activity of neighboring C sites. Increasing the amount of C-S species can further enhance the interaction of Hg with C sites, and leads to the formation of HgS. The adsorption of HgS on activated carbon surface is highly thermally favorable, and belongs to chemisorption. C-S species is able to facilitate the adsorption of HgS, and makes HgS adsorbing more likely in molecular adsorption pattern. HgS complex is the most stable form of Hg adsorption on activated carbon surface with H2S.