Membrane free volume is closely related to gas separation property. The free volume can be tuned by adjusting monomer structure for polyimide-based membrane materials, and then the gas separation property can be improved ulteriorly. In this work, crown ether (di(aminobenzo)-18-crown-6, DAB18C6)-based copolyimide membranes containing bulky and flexible diamine monomer (2,2'-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, HFBAPP) were synthesized for CO2 separation. The -C(CF3)(2) and -O- groups in HFBAPP can improve the membrane free volume and affinity with CO2, respectively. The microcavity size and fractional free volume of HFBAPP/DAB18C6/4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) membranes were investigated by molecular dynamics simulation and positron annihilation lifetime spectroscopy testing. The result showed that fractional free volume was improved obviously due to the presence of -C(CF3)2 in HFBAPP, while the microcavity size only showed very insignificant increase. Correspondingly, these membranes exhibited much higher CO2 permeability and satisfied selectivity. Furthermore, the correlation between free volume and gas transport properties of crown ether-based copolyimide membranes was investigated to establish structureproperty relationship. CO2/CH4 and CO2/N-2 mixed-gases separation properties of HFBAPP/DAB18C6/6FDA exceeded 2008 Robesons upper bounds.