HgCl2 that was homogenously dispersed over activated carbon was prepared by an HCl solution method (HgCl2(HCl)/AC), in the present study. The speciation of HgCl2(HCl)/AC in simulated flue gases was further investigated by a temperature-programmed decomposition desorption mass spectroscopy method, to clarify its transformation mechanism. It was found that HgCl2 sublimation occurred after Hg-H2O pretreatment, evidenced by the disappearance of the desorption peak of HgCl2 at 80 °C. After Hg-H2O pretreatment, new species of HgCl2 were formed, as confirmed by similar peak intensities of HgCl2 at mass numbers 270 and 272 at 335 °C. The SO2-H2O or H2S-H2O pretreatment helped stabilize the HgCl2 species supported over the AC, as evidenced by the weaker desorption peak of HgCl2 at around 80 °C. For the HgCl2(HCl)/AC pretreated with SO2-H2O, four desorption peaks of mercury were found, at 280, 335, 380 and 480 °C, indicating that new mercury compounds were formed during pretreatment or the TPDD process. The mercury desorbed at 280 °C was derived from a mercury complex of [HgCl]2SO4⋯A-C, while the mercury desorbed at 480 °C resulted from HgSO4. Some HgO species might be formed and stabilized over the AC during the TPDD process, corresponding to two mercury desorption peaks at 335 and 380 °C. After the pretreatment of HgCl2(HCl)/AC with H2S-H2O, some HgCl2 were transformed to HgS and HgO species, as evidenced by the mercury desorption peaks at 280, 305 and 380 °C.