The invention discloses a flue gas conditioning agent for flue gas mercury removal and a composite flue gas purifying method thereof. The flue gas conditioning agent is ammonium bromide powder or ammonium bromide solution. The flue gas conditioning agent is put into a flue of a combustion system at 150-450 DEG C and activated by use of the temperature of the flue gas; the activated flue gas conditioning agent reinforces conversion of the elemental mercury steam to mercury oxide and particle mercury in a flue gas environment, and the activation of the flue gas conditioning agent and the oxidation and adsorption of mercury are performed at the same time; and then the flue gas mercury removal is realized by use of the existing flue gas purifying equipment of the system. According to the flue gas conditioning agent and the composite flue gas purifying method disclosed by the invention, the flue gas is subjected to trace conditioning at specific temperature, and the gas-phase oxidation or gas-solid catalytic oxidation process of the mercury steam in the flue gas environment is reinforced so as to change the state of mercury in the flue gas; and finally, the flue gas mercury removal is realized by use of the existing purifying equipment of the system. The use of an activated carbon-based adsorbent can be avoided, and the flue gas mercury removal is realized by use of the existing equipment while maintaining the reusing value of fly ash.