CO2 emission has caused serious environmental problems, and the reduction of CO2 has become a global issue. The use of amine-impregnated solid adsorbents for CO2 capture from flue gas has been an effective approach. Herein, a novel porous MgCO3 was synthesized by a facile and template-free method and further impregnated with polyethylenimine (PEI) to prepare porous adsorbent for highly efficient CO2 capture. Moreover, during the synthesis of MgCO3, CO2 was used as one of the reactants in an attempt to improve the potential utilization of CO2. The effects of PEI loading, temperature, and coexisting gases (H2O, NO, and SO2) on CO2 adsorption performance were identified in a fixed bed. The results indicated that the adsorption capacity of the adsorbent toward CO2 was increased significantly after PEI impregnation. With the increase of temperature, the capture capacity of CO2 decreased because of the dominated thermodynamic control on the low PEI-loaded adsorbents, however, increased due to the governed kinetic control on the high PEI-loaded adsorbents. In addition, the adsorbent loaded with 20 wt % PEI showed the highest CO2 capture capacity, up to 1.07 mmol/g at 75 degrees C, and exhibited a 19.6% increase by introducing 10 vol % of water vapor. Meanwhile, the CO2 capture capacity was almost unaffected by NO; however, SO2 in flue gas could have some negative effects on CO2 adsorption due to the stronger acidity. Finally, the cyclic adsorption/desorption tests demonstrated excellent regenerability and stability of adsorbent. The results indicated that the PEI-modified porous MgCO3 is a very promising adsorbent for CO2 capture.