NOVELTY - Modified polyvinylidene fluoride electrolyte membrane is prepared by dissolving polyvinylidene fluoride in dimethyl sulfoxide, magnetically stirring to obtain colorless transparent solution, vacuum drying to obtain film (A), immersing in quat.-ammonium hydroxide alcohol solution to obtain dark modified polyvinylidene fluoride membrane (B), preparing styrene solution (C), adding modified polyvinylidene fluoride membrane (B) and initiator to styrene solution (C), swelling membrane, repeatedly washing to obtain grafted membrane (D), sulfonating, and washing with deionized water.    USE - Preparation of modified polyvinylidene fluoride electrolyte membrane for battery electrolyte.    ADVANTAGE - The preparation of modified polyvinylidene fluoride electrolyte membrane is suitable for industrialized production, yields product which is free of impurities and has good dimensional stability, high proton and electrical conductivity, high ion exchange capacity ratio, and low methanol permeability.    DETAILED DESCRIPTION - Modified polyvinylidene fluoride electrolyte membrane is prepared by dissolving polyvinylidene fluoride in 10-15 ml dimethyl sulfoxide, magnetically stirring in 80-100 degrees C oil bath for 0.5-1 hour to obtain colorless transparent solution, vacuum drying at 80-100 degrees C to obtain film (A), immersing in 0.1-0.5 mol/L quat.-ammonium hydroxide alcohol solution for 2-4 hours to obtain dark modified polyvinylidene fluoride membrane (B), preparing styrene solution (C) with volume ratio styrene:tetrahydrofuran which is 3-5:1, adding modified polyvinylidene fluoride membrane (B) and initiator to styrene solution (C), swelling membrane by adding methylene dichloride, repeatedly washing with deionized water to obtain grafted membrane (D) in which mass ratio of solutions (B):(C) is 0.1-0.5:1 and concentration of initiator is 0.3-0.5 g/100 ml solution C, sulfonating grafted membrane (D) in concentrated sulfuric acid at 70-80 degrees C for 2-4 hours, and washing with deionized water.