g-C3N4 nanosheets with tunable CO2 adsorption properties and nanostructures were synthesized and incorporated into polyether block amide (Pebax) membrane for CO2 separation. The g-C3N4 nanosheets with variable adsorption properties were produced from two monomers, dicyandiamide and melamine, and the variation of nanostructures was controlled by thermal oxidation etching process. The effects of CO2-philic and molecular sieving properties of g-C3N4 nanosheets on solubility and diffusivity of gas molecules in the as-prepared membranes were systematically investigated. The results demonstrated that the g-C3N4 nanosheets produced from dicyandiamide and undergoing 4 h thermal etching (DCN-4 nanosheets) showed the optimal CO2 sorption and sieving property. The membrane with 0.25 wt% DCN-4 nanosheets exhibited simultaneous enhancement in CO2 permeance and CO2/N2 selectivity compared with pure Pebax membrane. Moreover, the membrane maintained its separation performance during long-term operation test, showing great potential for CO2 capture.