Appropriate facilitated transport carriers and water environment are two principal factors for efficient CO2 membranes separation. In this study, the facile distillation-precipitation polymerization (DPP) and atom transfer radical polymerization (ATRP) methods are consecutively used to prepare polymer brushes functionalized double-shelled organic submicrocapsules, and the resultant organic submicrocapsules are incorporated into sulfonated poly(ether ether ketone) (SPEEK) to fabricate mixed matrix membranes (MMMs). The doubled shelled structure provides abundant facilitated carriers by the outer shell and high water retention property by the inner shell simultaneously. The organic submicrocapsules (b-IM@PMMA) are homogeneously embedded in the SPEEK matrix, and abundant amine groups on polymer brushes of outer shell increase the content of CO2-facilitated transport sites in MMMs. The inner shell renders the organic submicrocapsules of higher water content, yielding the MMMs with enhanced water retention properties. Moreover, the incorporation of organic submicrocapsules increases the water uptake and water retention capacity of MMMs. The MMMs doped with organic submicrocapsules display better CO2 separation performance than the MMMs doped with double shelled organic submicrocapsules without brushes. The polymer brushes functionalized double-shelled organic submicrocapsules act as water reservoirs to not only offer more water for the dissolution of CO2 gas molecules, but also construct and widen interconnected continuous CO2-facilitated transport "passageways. The highest CO2/CH4(N-2) selectivity is 73.8(76.3) for the SPEEK/b-IM@PMAA MMMs (at a CO2 permeability of 2236 Barrer) in pure gas, surpassing the 2008 Robeson upper bound.