With the citric acid preparation route, CaO–Al2O3 sorbents with different amount of Al2O3 additive were synthesized. Then the synthetic sorbent materials were activated with a four-step heating mode, and the mild calcination activation conditions might favor the formation of porous structure in the synthetic sorbents. CaO–Al2O3 sorbent with 9% Al2O3 additive has higher BET surface area (958 m2/g) and larger pore volume (0.85 cm3/g, BJH), and the particle structure is more porous and sponge-like, many nano-sized CaO grains are distributed on the porous surface of sorbent. It is found that the stable inert support substance Ca3Al2O6 is formed in the synthetic CaO–Al2O3 sorbent after a few carbonation/calcination cycles, which provides a stable framework to inhibit the severe sintering of CaO particles. With the developed porous structure and the existence of inert support substance to prevent the sintering of CaO particles, the CO2 capture capacity, reaction kinetics and long-term cyclic stability of the synthetic CaO–Al2O3 sorbent are improved significantly when compared with the performance of untreated CaO.