The idea of applying the gas–liquid–solid (GLS) three-phase circulating fluidized bed to CO2 capture by aqueous ammonia is proposed in this work. A three-phase circulating fluidized bed test rig was built, and a series of experiments were carried out to study the multiphase flow and gas–liquid mass transfer behavior. Compared with the conventional reactors, the solids in the three-phase circulating fluidized bed can break up bubbles into much smaller ones and distribute them more uniformly. The experimental results show a three-phase circulating fluidized bed can provide larger interface area, longer residence time, and higher mass transfer efficiency. Based on these results, the process for CO2 capture by aqueous ammonia using a three-phase circulating fluidized bed was designed conceptually and its feasibility was studied preliminarily by Apsen Plus—a simulation software. When the mole fraction of aqueous ammonia is about 5.16% in the three-phase circulating fluidized bed, more than 90% of the CO2 can be absorbed and the ammonia escaping ratio is very small. The three-phase circulating fluidized bed presents great potential in CO2 separation by aqueous ammonia.