Surface modification by generating a dense anionic layer on the surface of a substrate membrane is the main method for preparing monovalent selective anion exchange membranes. However, this method has a trade-off between the permselectivity and the stability of the surface layer. To overcome this problem, a simple preparative pathway is devised to covalently immobilize the polyethyleneimine (PEI) onto the surface of an anion exchange membrane (partly quaternized poly (phenylene oxide) (QPPO)). Firstly, a brominated poly (2,6-dimethyl-1, 4-phenylene oxide) (BPPO) based anion membrane was prepared by quaternization. Then, this membrane was modified by electro-deposition of a PEI solution. The composition and structure of the membrane were observed by infrared spectroscopy and scanning electron microscopy. The permselectivity was evaluated by electrodialysis in a Cl-/SO42- system. The experimental results prove that the PEI modification method is an effective, simple, feasible and well-controlled strategy to fabricate mono-valent selective anion membranes.