In order to enhance wet flue gas desulfurization efficiency and reduce cost, the effects of different series of additives (typical organic acids, organic salts, inorganic salts, organic amines, and their mixture) on the desulfurization properties, including pH buffering characteristics of slurry, dissolution characteristics of CaCO3, oxidation characteristics of CaSO3, desulfurization efficiency, and economic cost were investigated in this work. The results show that the pH buffering capacity is sodium benzoate approximate to Na2SO4 > adipic acid > double oxazine. The dissolution promoting effect of CaCO3 is acetic acid > double oxazine approximate to sodium acetate > MgSO4. The additives that can significantly promote the oxidation of CaSO (3) are adipic acid, citric acid, acetic acid, MgSO4, and Na2SO4. Without an additive, the desulfurization efficiency of CaCO3 slurry decreased to 85.87% after 4 h. After the addition of the composite additive (CA-3), the desulfurization efficiency of CaCO3 slurry remained 96.53% for 4 h, increasing by 10.66%. The main reasons for the enhancement in the desulfurization process may be a collection of three factors: the reversible hydrolysis reactions of acetic acid and sodium acetate, the formation of an acid-base conjugate system by sodium benzoate, and the formation of a neutral ion pair MgSO3 by MgSO4 in the slurry. In addition, the composite additive has been used in the Dabie mountain power plant of China. The economic benefit analysis shows that the desulfurization unit of 4 X 640 MW coal-fired power plant can save 1.26 million dollars per year by composite additive.