TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The liquid flow battery diaphragm has an ion exchange capacity of 0.8-1.5 mmol/g and a film thickness of 20-200 mu m.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: In the step (i), the solvent is at least one of 1,1,2-trichloro-1,2,2-trifluoroethane, N,N-dimethylformamide, dimethylacetamide, N-methylformamide, dimethyl sulfoxide, N-methyl pyrrolidone, hexamethyl phosphoramide, acetone, water, ethanol, methanol, propanol, isopropanol, glycol or glycerol. In the step (iii), the crosslinking agent is at least one of ammonia, amine compound of formula (H2NRxNH2) (VII) and an organic diamine, or the chemical treatment agent that can release ammonia, hydrazine or an organic diamine, preferably ammonia, hydrazine, perfluoromethylenediamine or ethylenediamine. The organic diamine is 1-10C alkyl or fluoroalkyl diamine. The chemical treatment agent is ammonia, hydrazine, organic or inorganic acid salts of the organic diamine, urea, or guanidine.x = 0-10.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The perfluorosulfonic acid resin used for film formation is obtained by polymerizing 1,1,2,2-tetrafluoro-2-(1,1,1,2,3,3-hexafluoro-3-(1,2,2-trifluorovinyloxy)propan-2-yloxy)ethanesulfonyl fluoride (III), 1,1,2,2,3,3-hexafluoro-3-(1,2,2-trifluorovinyloxy)propane-1-sulfonyl fluoride (IV), 1,1,2,2-tetrafluoro-2-(1,2,2-trifluorovinyloxy)ethanesulfonyl fluoride (V) and/or 1,1,2,2,3,3,4,4-octafluoro-4-(1,2,2-trifluorovinyloxy)butane-1-sulfonyl fluoride (VI), and carrying out acid hydrolysis of the obtained polymer, where the molecular weight of the polymer is 140000-500000. Preferred Method: The preparation of the liquid flow battery diaphragm comprises: either (i) dispersing the perfluorosulfonic acid in a solvent to obtain a dispersion liquid having a solid content of 1-80 wt.%, (ii) forming the film by applying the dispersion liquid on a flat plate by using solution casting, solution pouring, screen painting, spin coating, spray finishing or dipping technology, and heat treating at 30-300 degrees C for 10-100 minutes, (iii) separating the film from the flat plate, applying the solution of crosslinking agent or crosslinking agent on one or both the surfaces of the film by spraying, dipping or coating method, allowing to stand for 1-20 minutes to allow the cross-linking agent to penetrate the membrane surface layer, and (iv) heat treating the film obtained in the step (iii) at 30-300 degrees C; or (i) mixing the perfluorosulfonic acid resin in a twin-screw extruder, internal mixer or a mill at 100-280 degrees C; (ii) mixing resin by using a screw extruder or flat vulcanizing machine and film forming, (iii) applying the crosslinking agent or its solution on the film surface by spraying, spin coating method, allowing to stand for 1-20 minutes, (iv) heating the film at 30-300 degrees C, and (v) hydrolyzing and acidifying film formed in step (iv) by using alkali liquid.