NOVELTY - A sulfonated polybenzimidazole copolymer (I) is new.    USE - Sulfonated polybenzimidazole copolymer used for preparing sulfonated polybenzimidazole crosslinking membrane used in all-vanadium liquid flow battery and proton exchanging film fuel battery (all claimed).    ADVANTAGE - The membrane has good machine performance, high ion exchange capacity, excellent chemical stability and low vanadium ion transmission rate. The all-vanadium liquid flow battery has high coulomb efficiency and energy efficiency, low automatic discharge circuit speed and good charge-discharge circuit and circulation performance.    DETAILED DESCRIPTION - A sulfonated polybenzimidazole copolymer of formula (I) is new.    R1=group of formula (i)-(viii);    R2=-O- and S(=O);    a, b=0 or 1;    c=0-4; and    x=0-1.    INDEPENDENT CLAIMS are included for:    (1) preparation of compound (I) copolymer;    (2) sulfonated polybenzimidazole crosslinking membrane comprising compound (I); and    (3) preparation of sulfonated polybenzimidazole crosslinking membrane comprising: (A) dissolving aromatic tetramine, aromatic dicarboxylic acid and amino acid into polyphosphoric acid under nitrogen protection, reacting at 150-250 degrees C for 2-30 hours, cooling to 80-120 degrees C, pouring into ice water, suction filtering, washing filter cake with deionized water, and steeping in 5% sodium bicarbonate solution for 1-2 days; (B) suction filtering, washing filter cake with deionized water to neutral, vacuum drying, dissolving into concentrated sulfuric acid or fuming sulfuric acid under nitrogen protection, heating to 50-120 degrees C, reacting for 1-30 hours, cooling to room temperature, pouring into ice water, suction filtering, leaching filter cake into 5% sodium bicarbonate solution for 1-2 days, and suction filtering; and (C) washing filter cake with deionized water to neutral, and vacuum drying, dissolving into organic solvent, adding crosslinking agent, uniformly mixing, casting on clean glass plate, placing into drying box for drying at 60-120 degrees C for 2-10 hours.