Sulfur impregnated activated carbon has been proven to be promising adsorbents for mercury (Hg0) removal in the flue gas. However, the active sites and detailed reaction pathway happened on the carbonaceous surface still remain uncertain. Here, we investigated the interaction behavior of various Sn allotropes (n = 2–8) with carbonaceous surface and the subsequent Hg0 adsorption process by quantum chemistry calculation. The results indicate all Sn molecules could be attached to the carbonaceous surface by chemisorption with calculated adsorption energy lower than −400 kJ/mol. Moreover, the Hg0 binding on virgin activated carbon surface takes place by physisorption with calculated adsorption energy around −20 kJ/mol. Sn allotropes (n = 2–3,6–8) attached on the carbonaceous surface have a negligible influence on improving Hg0 removal performance. In contrast, S4 and S5 attached to the carbonaceous surface have a positive effect on improving Hg0 adsorption performance and form S-Hg and C-Hg bonds in the products. Moreover, electronic wavefunction analysis was also conducted to investigate chemical bonds breaking and formation process in this research. Both bond lengths analysis and Mayer bond order value analysis were employed and localized molecular orbital analysis results indicate the formed S-Hg and C-Hg bonds exhibit typical σ bond character in the products. In addition, steam, a major component in the flue gas, could greatly accelerate the Hg0 adsorption on carbonaceous surface by lowering reaction energy barriers. This research would be helpful for the development of efficient sulfur impregnated carbon based adsorbents in the future.