Abstract: Polyvinylpyrrolidone(PVP)was synthesized by the usual radical polymerization, and the composite membrane was prepared with PVP hydrolysate poly{N-vinyl-γ-sodium aminobutyrate}(PVSA) and polysulfone ultrafiltration membrane as its support. The structures of PVP and PVSA were investigated by infrared spectrum (IR) and X-ray diffraction (XRD).The permselectivities of the composite membrane for CO₂/CH₄ system were measured.The results show that at 26℃，1333Pa of gas pressure, the membrane displays a CO₂ permeation rate of 5.46×10^-7 cm³(STP)•cm^-2•s^-1•Pa^-1 and CO₂/CH₄ ideal selectivity of 212.1. With increasing pressure in feed gas, both CO₂ permeation rate and selectivity of CO₂/CH₄ decrease. The facilitated transport mechanism of CO₂ in membrane was analyzed according to the characteristics of permeation flux vs pressure and the infrared spectra of the membranes in different states.