Carbon capture and storage (CCS) is a promising strategy for reduction of CO₂ emissions. Herein, CO₂/N₂ adsorption and separation in three SIFSIX-X-Cu (arrayed via SiF₆^2- with Cu metal center, X = 2, 3, O) hybrid ultramicroporous materials at 298 K within 0-5 kPa were investigated by using grand canonical Monte Carlo (GCMC) simulation. Results showed that, in contrast to SIFSIX-2-Cu, CO₂ adsorption in SIFSIX-3-Cu and SIFSIX-O-Cu reached saturation at 0.5 kPa and their CO₂ adsorption capacity were 2.70 and 2.39 mmol·g ^-1 at 1 kPa, respectively. The CO₂ adsorption capacity in CO₂/N₂ mixture barely decreased. SIFSIX-3-Cu and SIFSIX-O-Cu owned close pore sizes to CO₂ dynamics diameter, thereby exhibiting high CO₂ affinity with adsorption heat of 59 and 66 kJ·mol ^-1, respectively. Density functional theory (DFT) analyses showed only one CO₂ molecule could be adsorbed in each hole and located at the center of SIFSIX-3-Cu and SIFSIX-O-Cu. Our results provide a theoretical guidance for developing ultramicroporous materials in adsorption and separation of CO₂ at low pressure.