The change of mercury speciation was measured by a continuous emission monitor when elemental mercury (Hg degrees) was adsorbed by activated carbon (AC-CS, made from coconut shell) in simulated coal-fired flue gas on a bench-scale adsorption experiment. Results showed that Hg degrees was oxidized to divalent mercury (Hg2+) and escaped as Hg2+ completely during the adsorption process in simulated flue gas. The components, such as NO, NO2, etc., had double influences on the Hg degrees adsorption on AC-CS. The positive way was that they could oxidize Hg degrees to Hg2+ with the catalysis of AC-CS surface, which promotes the Hg degrees adsorption, and the negative way was that they had competitive adsorption with Hg2+ on the AC-CS surface to inhibit the adsorption to a certain degree. It is economical to use the original AC to capture Hg in a complete flue gas with oxidization components and to use chemically treated AC in a clean flue gas.