Abstract: The SO₃ generated during coal combustion in thermal power plants not only damages equipment and catalysts in selective catalytic reduction (SCR) systems but also poses significant health risks. Therefore, the removal of SO₃ from coal-fired flue gas is of great importance. The low-particle-size calcium hydroxide was synthesized using the inhibition method, and the effects of different additives on the crystal growth of calcium hydroxide were thoroughly investigated. The impact of various factors such as the absorbent particle size, initial SO₃ concentration, m(Ca)/m(S), absorbent residence time, and flue gas temperature on SO₃ removal efficiency was systematically analyzed. The reaction mechanism was also further explored. Results show that the calcium hydroxide-sodium benzoate absorbent, which has the smallest and most uniform particle size, achieves the highest SO₃ removal efficiency and offers good economic viability. The absorbent's residence time, m(Ca)/m(S), initial SO₃ concentration, and flue gas temperature can significantly influence SO₃ removal efficiency. During the reaction, the product layer exhibits an island growth effect, and the Bangham model is found to describe the adsorption kinetics of SO₃ effectively.