A flue gas treatment system and a flue gas treatment method capable of drastically reducing operating cost and an amount of waste water and capable of controlling an amount of a NOx reducing agent and a mercury oxidation agent to be supplied are provided. The flue gas treatment system sprays an aqueous solution containing ammonium halide into a flue, reduces NOx and oxidizes mercury in a deNOx section, and removes SOx and the mercury in a desulfurization section. The flue gas treatment system adds at least one of ammonium sulfate and ammonium carbonate to waste water which is discharged from the desulfurization section and from which CaSO4 is separated to generate the ammonium halide. The waste water containing the ammonium halide is sprayed as the aqueous solution.