Developing novel adsorbents for CO2 capture and separation offers an efficient approach to relieving global environmental problems. To promote CO2 capture from flue gas, novel Cr3O@MIL-101(Cr) composites was synthesized through packing [Cr3O(HCOO)6(H2O)3]NO3 into MIL-101(Cr) cages. The resultant composite Cr3O@MIL-101(Cr) presents higher CO2 uptakes at lower pressure ranges, benefiting from the increased content of unsaturated Cr(III) ions and micropores relative to MIL-101(Cr). Meanwhile, IAST selectivity for CO2/N2 mixture reached 50.2 under standard temperature and pressure, outperforming most of the reported materials. Further GCMC simulations reveal that the synergistic effect of [Cr3O(HCOO)6(H2O)3]+, NO3− and MIL-101 (Cr) could effectively facilitate the interactions with CO2 molecules and hinder N2 molecules from approaching the optimal adsorption sites. Thus the [Cr3O(HCOO)6(H2O)3]NO3 encapsulated MIL-101(Cr) can be recommended as a promising adsorbent for superior CO2/N2 separation.