Ammonia has proved an alternative and potential solvent for postcombustion CO2 capture. It is of benefit to the reduction of energy consumption for CO2 capture to clarify the CO2 absorption characteristics in ammonia solution. On the basis of the representative elementary volume method and pseudo-single-liquid model, the computational model of CO2 absorption into ammonia solution in the structured packed column of a pilot CCS plant was developed, in which the gas-phase flow was also taken into account. The modeling and computational methods were validated by comparing with, the experimental data from two pilot CO2 absorption towers. The CO2 removal efficiency and volumetric overall mass transfer coefficient were obtained at various inlet CO2 volume fractions, gas flow rates, aqueous ammonia solution flow rates and temperatures, and the ammonia mass fractions. The results show that the absorption process is predominated by the liquid film, so the flow rate and ammonia concentration of the aqueous ammonia solution play important roles in the mass transfer capacity of CO2.