NOVELTY - Imidazolyl-containing sulfonated polyimide covalent-ionic crosslinked membrane is prepared by reacting diamine monomer, triethylamine, m-cresol, 1,4,5,8-naphthalenetetracarboxylic dianhydride and phenylformic acid with nitrogen protection, cooling, precipitating and washing with acetone, drying precipitate, dissolving in organic solvent, casting on glass or stainless steel plate, drying, soaking formed film in methanol or acetone and hydrochloric or sulfuric acid, washing, drying and soaking in methanesulfonic acid/phosphorus pentoxide solution, or phosphoric or polyphosphoric acid solution.    USE - Method of preparing imidazolyl-containing sulfonated polyimide covalent-ionic crosslinked membrane (claimed) used for fuel cell.    ADVANTAGE - The method prepares membrane having superior property in resisting free-radical oxidation compared to imidazolyl-free sulfonated polyimide crosslinked or noncovalently crosslinked membrane.    DETAILED DESCRIPTION - Imidazolyl-containing sulfonated polyimide covalent-ionic crosslinked membrane is prepared by:    (A) mixing diamine monomer comprising (mol%) sulfonated diamine (5-80), non-sulfonated diamine (2-30) and imidazolyl-containing diamine (18-90), triethylamine, m-cresol, 1,4,5,8-naphthalenetetracarboxylic dianhydride and phenylformic acid with nitrogen protection at 1,4,5,8-naphthalenetetracarboxylic dianhydride to diamine monomers molar ratio of 1:1, 1,4,5,8-naphthalenetetracarboxylic dianhydride to phenylformic acid molar ratio of 1:1-2, sulfonated diamine to triethylamine molar ratio of 1:2-4 and total 1,4,5,8-naphthalenetetracarboxylic dianhydride and diamine monomer concentration in m-cresol of 5-40 wt.%;    (B) reacting at 80 degrees C for 4 hours, reacting at 180 degrees C for 10-20 hours, cooling, pouring into acetone, washing obtained thread-like precipitate with acetone for 1-3 times and drying in vacuum to obtain imidazolyl-containing sulfonated polyimide;    (C) dissolving obtained imidazolyl-containing sulfonated polyimide (1-40 wt.%) in organic solvent, casting on glass or stainless steel plate, drying at 60-150 degrees C for 2-20 hours, peeling off formed film from plate, soaking in methanol or acetone for 1-10 hours and 0.01-1 M hydrochloric or sulfuric acid for 5-50 hours, washing with deionized water until neutralized and drying in vacuum at 50-150 degrees C for 1-30 hours to obtain unshared crosslinked imidazolyl-containing sulfonated polyimide membrane;    (D) soaking unshared crosslinked imidazolyl-containing sulfonated polyimide membrane in mixture of methanesulfonic acid/phosphorus pentoxide solution at weight ratio of 100:1-20 and 25-150 degrees C for 0.1-50 hours, or in 50-100 wt.% phosphoric acid solution for 0.1-50 hours and drying in vacuum at 130-200 degrees C for 2-50 hours, or in polyphosphoric acid solution comprising 10-90 wt.% phosphorus pentoxide at 100-200 degrees C for 1-50 hours; and    (E) washing with deionized water until neutralized and drying in vacuum.