Recently, petroleum coke has been developed to remove elemental mercury (H-g(0)) in flue gas as a carbon-based adsorbent. Brominated modified petroleum coke with a new bromine modification method showed promising results for capturing H-g(0). Revealing the mechanism of mercury adsorption processes on petroleum coke and brominated modified petroleum coke is crucial to the designing and development of cheap and efficient adsorbents. To simulate the different edges of petroleum coke, six different cluster models were established and verified. The possible reaction paths of HgO, HgBr, and HgBr2 formation on petroleum coke were presented, and the binding energies were given. The results revealed that HgO, HgBr, and HgBr2 were bound on the surface of brominated modified petroleum coke and HgBr was most likely to be generated. The effects of S, N, O, and Br in the brominated modified petroleum coke on elemental mercury adsorption were also revealed, which could be a reference to mercury capture efficiency of brominated modified petroleum with different S, N, O, and Br contents.