Alkaline injection is an effective way to control the emission of high concentration SO3 caused by the use of high sulfur coal and selective catalytic reduction (SCR), but its performance is affected by the competitive absorption of SO2. A fixed-bed reactor was used to study the competitive absorption of SO3 and SO2 by Ca(OH)(2). The dynamic absorption curves in different atmospheres were obtained, the intermediate products were characterized, and the competitive absorption mechanism was discussed. The absorption of SO3 and SO2 consists of the chemical kinetics-controlled and product layer diffusion-controlled stages. In the chemical kinetics-controlled stage, the SO3 and SO2 removal efficiencies were around 90% and 55% at first and then decreased; the SO3 selectivity was around 0.1 at first and then increased. In the product layer diffusion-controlled stage, the SO3 removal efficiency gradually decreased but kept greater than 10%, while the SO2 removal efficiency deceased to zero; the SO3 selectivity continued to increase and reached 0.31 at most when SO2 absorption ceased. There existed significant competition between SO3 and SO2 absorption. The increasing SO3 concentration promoted the SO3 absorption and the increasing SO2 concentration decreased it. The absorption of SO3 increased the intensity of the SO42- band, decrease the intensity of the se band, and made the product layer more compact. The possibility of SO3 reacting with the components (CaSO3, CaSO4 and CaCO3) on the reacted Ca(OH)(2) surface was analyzed. Ionic diffusion was proposed as the main mechanism of the continuous SO3 absorption after the formation of a dense product layer.