In this study, functional polyethersulfone (PES) hollow fiber membranes with pH sensitivity were prepared by blending with poly(acrylic acid-co-acrylonitrile) (P(AA-AN)) copolymer. The copolymer was characterized by FTIR analysis, elemental analysis, and GPC measurement. Scanning electron microscopy (SEM) was used to investigate the morphology of the blended hollow fiber membranes. The modified hollow fiber membranes showed excellent pH sensitivity and pH reversibility and we confirmed that both the pore size change and the electroviscous effect had great effect on the pH sensitivity of the copolymer blended PES hollow fiber membranes.