Ionic liquid (IL)/polyimide (PI) composite membranes demonstrate promise for use in CO separation applications. However, few studies have focused on the microscopic mechanism of CO in these composite systems, which is important information for designing new membranes. In this work, a series of systems of CO in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide composited with 4,4-(hexafluoroisopropylidene) diphthalic anhydride (6FDA)-based PI, 6FDA-2,3,5,6-tetramethyl-1,4-phenylene-diamine, at different IL concentrations were investigated by all-atom molecular dynamics simulation. The formation of IL regions in PI was found, and the IL regions gradually became continuous channels with increasing IL concentrations. The analysis of the radial distribution functions and hydrogen bond numbers demonstrated that PI had a stronger interaction with cations than anions. However, the hydrogen bonds among PI chains were destroyed by the addition of IL, which was favorable for transporting CO . Furthermore, the self-diffusion coefficient and free energy barrier suggested that the diffusion coefficient of CO decreased with increasing IL concentrations up to 35 wt-% due to the decrease of the fractional free volume of the composite membrane. However, the CO self-diffusion coefficients increased when the IL contents were higher than 35 wt-%, which was attributed to the formation of continuous IL domain that benefitted the transportation of CO .