Adsorbents with high adsorption capacity and high selectivity are crucial for carbon capture, utilization and storage (CCUS) technology applied to industrial flue gas to reduce carbon emissions. In this paper, a defect-rich hierarchical porous Mg-MOF-74 was synthesized successfully with MgCl₂·6H₂O used as a metal source considering that the weak coordination of Cl⁻ may interfere with the normal coordination process of metal centers and organic ligands, which causes the formation of crystal defects. Through the analysis of a series of characterizations, ligand deletion of Mg-MOF-74 prepared with MgCl₂·6H₂O was confirmed, and the deleted ligand was estimated to be approximately 20% compared with the traditional Mg-MOF-74 prepared with Mg(NO₃)₂·6H₂O, which led to mesopores accounting for 36.9%, and the width of mesopores reached 13.1 nm. Compared with traditional Mg-MOF-74, the adsorption heat of CO₂ at zero load of the defect-rich hierarchical porous Mg-MOF-74 increased from 36 kJ·mol⁻¹ to 46 kJ·mol⁻¹, and the saturated adsorption capacity of CO₂ under ambient pressure was improved by 15%. Interestingly, the IAST selective adsorption performance of CO₂/N₂ was increased by 20 times. Compared with the saturated adsorption capacity for traditional Mg-MOF-74, the equivalent adsorption capacity of hierarchical porous Mg-MOF-74 only needs 17 kPa, indicating that it is suitable for carbon capture operation in a real flue environment. Calculations of density functional theory (DFT) confirmed that the active site of the hierarchical porous Mg-MOF-74 has a lower adsorption energy with CO₂ due to the presence of defects, which is regarded to play a key role in improving the CO₂ capture performance.