Abstract: Low temperature NH3 plasma treatment was applied to introduce N-contained hydrophilic groups onto the surface of chitosan polyacrylonitrile composite membranes. The modified membranes were positively charged in weak acid solutions under low pressure. Effect of the plasma treatment time and discharge power on the improving of the film hydrophilic was discussed. The best plasma processing conditions were determined by single factor experiment. SPM, water contact angle and XPS measurements were carried out to characterize surface changes. After plasma treatment, both permeation and filtration performance were greatly improved. Under pressure 0.05 MPa and pH≈6.7, the solution flux was 3.19 L/(m2·h) and the rejection rate for γ-aminobutyric acid was 78%.