NOVELTY - A polybenzimidazole binding agent is prepared by performing condensation polymerization reaction of 3,3',4,4'-tetra-aminobiphenyl and isophthalic acid in the presence of polyphosphoric acid containing 70-90 wt.% of phosphorus pentoxide to obtain polybenzimidazole binding agent having a molecular weight of 5000-16000. The molar ratio of 3,3',4,4'-tetra-aminobiphenyl to isophthalic acid is (1:1.02)-(1:1.06). The total mass concentration of 3,3',4,4'-tetra-aminobiphenyl and the isophthalic acid is 5-20 wt.%.    USE - Method for preparing polybenzimidazole binding agent for fuel battery film electrode (claimed) which can be used for generating electricity of, e.g. hydrogen-oxygen fuel battery and oxygen-air fuel battery.    ADVANTAGE - The method is simple, needs no co-solvent, and provides a polybenzimidazole binding agent having high binding performance and which can be dissolved easily in multiple aprotic solvents. The fuel battery film electrode incorporating the polybenzimidazole binding agent can be utilized for generating electricity of hydrogen-oxygen fuel battery and oxygen-air fuel battery without humidifying at higher than 100 degrees C, with the temperature of the hydrogen-oxygen fuel battery at 160 degrees C and with its maximum power density without humidifying of 366 mW/cm2 (976 mA/cm2, 0.375 V).    DETAILED DESCRIPTION - A polybenzimidazole binding agent is prepared by adding 3,3',4,4'-tetra-aminobiphenyl and isophthalic acid to polyphosphoric acid containing 70-90 wt.% of phosphorus pentoxide; reacting for 5-30 hours at 50-220 degrees C under protection of nitrogen; cooling to 10-40 degrees C; precipitating to the water; neutralizing with alkali, filtering, washing, and drying to constant weight in vacuum at 100 degrees C to obtain the polybenzimidazole binding agent having a molecular weight of 5000-16000. The molar ratio of 3,3',4,4'-tetra-aminobiphenyl to isophthalic acid is (1:1.02)-(1:1.06). The total mass concentration of 3,3',4,4'-tetra-aminobiphenyl and isophthalic acid is 5-20 wt.%. An INDEPENDENT CLAIM is included for application of the polybenzimidazole binding agent in fuel battery film electrode comprising (1) dissolving polybenzimidazole binding agent (1 mass part) in organic solvent (100-200 mass parts); (2) adding platinum-carbon catalyst (3-20 mass parts), stirring it and preparing an electrode slurry ultrasonically; (3) coating the electrode slurry onto an air diffusing layer, drying the mixture to constant weight in vacuum at 190 degrees C to obtain an air-diffused electrode; (4) soaking the air-diffused electrode in an alcohol solution containing 15 wt.% phosphoric acid, dipping the electrode for 2-30 minutes; (5) drying the electrode to constant weight at 60-80 degrees C to obtain the phosphoric acid-doping air-diffused electrode; (6) placing one side containing the platinum-carbon catalyst of each of two phosphoric acid-doping air-diffused electrodes toward a high-temperature proton exchange film; and (7) thermally pressing for 1-20 minutes at 100-160 degrees C and 1-4 MPa to obtain a polybenzimidazole binding agent for the fuel battery film electrode. The volume of the alcohol solution needed by the air-diffused electrode in step (4) is 1-5 mL/cm2.