The disclosed forms a proton exchange membrane. First, multi-maleimide and barbituric acid are copolymerized to form a hyper-branched polymer. Next, the solvent of the sulfonated tetrafluorethylene copolymer (Nafion) aqueous solution is replaced from water with dimethyl acetamide (DMAc). 10 to 20 parts by weight of the hyper-branched polymer is added to the 90 to 80 parts by weight of the Nafion in a DMAc solution, stood and heated to 50° C. to inter-penetrate the hyper-branched polymer and the Nafion. The heated solution is coated on a substrate, baked, and pre-treated to remove residue solvent for completing an inter-penetrated proton exchange membrane.