The invention relates to an additive for inhibiting re-release of mercury during wet method flue gas desulfurization process and a using method thereof. The additive comprises the following components in percentage by weight: 30%-45% of sodium tetrasulfide, 30%-45% of dimethyl dithiocarbamate and 10%-40% of trimercapto-triazine metal salt. At normal temperature and normal pressure, the additive is added into desulfurization slurry, and the adding amount of the additive is 50-800mg/L. The additive disclosed by the invention can perform better action with Hg2+ in the slurry by utilizing the synergistic effect of the sodium tetrasulfide, the dimethyl dithiocarbamate and the trimercapto-triazine metal salt so as to form a stable HgS precipitate or a stable chelate, and the precipitate or the stable chelate and desulfurization gypsum can enter a solid phase together for being separated and removed. Simultaneously, the composite additive is more suitable for being added under operation conditions of a wet method desulfurization system.