To improve the film-forming property and gas permeability of poly(ionic liquid) membranes, a series of semi interpenetrating polymer network (semi-IPN) membranes were successfully synthesized by incorporating linear polyvinyl acetate (PVAc) into a cross-linked poly (ionic liquid) (c-PIL) network. The resultant PVAc/c-PIL semiIPN membranes exhibited good film-forming property and various phase separation morphologies, from spherical, interconnected to co-continuous microstructure. Also, the formation of semi-IPN structure endowed the membrane with an improved mechanical properties and slightly depressed glass transition temperatures. From gas permeation measurement results, PVAc/c-PIL semi-IPN membranes demonstrated gradually higher CO2 permeability coefficient with the increase of c-PIL content, while CO2/N-2 permselectivity increased initially and then decreased slightly. The best permeability and permselectivity of the membrane reached 36.1 Barrer and 59.6, respectively, when the c-PIL content was 50 wt%, which approached to the 2008 Robeson limit. Moreover, the permeability of CO2 in PVAc/c-PIL-50 semi-IPN membrane increased with increasing temperature across their glass transition temperatures, while the permselectivity of CO2/N-2 decreased.