Based on a comprehensive study of the pyrolysis characteristics, pore structure, surface chemical functional groups, organic carbon framework structure, microcrystalline morphology, and lattice characteristics of walnut shell biochar for mercury adsorption, the structural characteristics of biochar were analyzed, and a model of the monomeric molecular structure of biochar was constructed. In addition, the density functional theory of quantum mechanics was introduced into the gas-solid adsorption reaction of elemental mercury with biochar. By quantitatively studying the adsorption energy, adsorption height, and Mulliken population number of the adsorption system, the adsorption mechanism of Hg-0 by biochar was revealed to provide a theoretical basis for future mercury removal methods. The results showed that walnut shell biochar consisted mainly of C, H, O, and N. Aromatic carbon was the main component of the biochar molecular structure, while aliphatic carbon was linked to aromatic structural units. The molecular structure model was mainly composed of aromatic structures, including one methyl, four hydroxyl, and eight carbonyl groups. The molecular formula was C55H37NO14, and the molecular weight was 935. The adsorption of Hg-0 by biochar was mainly via chemical adsorption, and the adsorbed Hg-0 could stably exist on the biochar surface. The Hg-0 adsorption process on the biochar surface mainly depended on the charge of the adsorption site. An adsorption site with a negative charge and with a high number of charges facilitated the adsorption of elemental mercury on the biochar surface. Moreover, the charge of the ortho atom adjacent to the adsorption site had a substantial influence on the adsorption activity of the adsorption site.