The development of cation-exchange membrane with the ability to effectively separate mono/divalent ions is of crucial importance to various industrial applications such as wastewater purification and seawater desalination. However, it remains a big challenge to fabricate monovalent cation selective membrane maintaining both of high ion flux and good permselectivity. In this study, nanofibrous composite membranes (NFCMs) containing -N+(CH3)(3) and -SO3- groups are fabricated by impregnating bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) electrospun nanofibrous mats into sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) solution and followed by quaternization of the bromomethyl groups. The unique nanofibrous composite structure results in low electrical resistance, high limiting current density (jam) and significant improvement in dimensional stability and ion flux. In addition, due to the introduced quaternized electrospun nanofibrous mats, monovalent cations can be separated from divalent cations by their difference of electrostatic repulsion force. Compared with the commercial monovalent cation selective membrane (CSO), the optimized membrane (NQS1) in this study shows better performance with the ion flux of 2.96 x10(-8) mol cm(-2) s(-1) and the permselectivity of 1.62 in an electrodialysis process (feed solution: 0.1 mol L-1 NaCl/MgCl2). According to the excellent performances, our nanofibrous composite membranes are expected to be a promising candidate for Na+/Mg2+ separation.