NOVELTY - A polyvinylidene fluoride porous membrane is prepared by placing methyl methacrylate monomer, chain transfer agent and azo initiator in reactor, adding solvent, placing into oil bath, placing reaction bottle into ice/water mixture for quenching, washing deposits, and vacuum drying to obtain polymethyl methacrylate macromolecular chain transfer agent; dissolving chain transfer agent, polymeric halogenated imidazole ionic liquid and azo nitrile in dimethyl formamide; and mixing polyionic liquid brush copolymer, polyvinylidene fluoride, pore former and solvent to obtain film-casting solution.    USE - Method for preparing polyvinylidene fluoride porous membrane (claimed), for water treatment, chemical sensor, and fuel cell.    ADVANTAGE - The method is capable of providing film with ion sensitivity.    DETAILED DESCRIPTION - Preparing polyvinylidene fluoride porous membrane comprises:    (A) placing methyl methacrylate monomer, chain transfer agent and azo initiator in reactor, adding solvent, mixing and stirring under nitrogen gas protection, placing into 60-70 degrees C oil bath to react for 12-20 hours, placing reaction bottle into ice/water mixture for quenching to stop the reaction, washing deposits with methanol for 2-5 times, and vacuum drying to constant weight to obtain pale yellow polymethyl methacrylate macromolecular chain transfer agent;    (B) dissolving polymethyl methacrylate macromolecular chain transfer agent, polymeric halogenated imidazole ionic liquid and azo nitrile in dimethyl formamide, introducing nitrogen in reaction bottle, placing in 60-70 degrees C oil bath to react for 12-24 hours, quenching and precipitating, and drying the precipitate at 40 degrees C for 48 hours in vacuum to obtain block charged polyionic liquid brush copolymer; and    (C) mixing polyionic liquid brush copolymer, polyvinylidene fluoride, pore former and solvent to obtain film-casting solution, reacting at 60-70 degrees C for 12-24 hours, static deaerating, scraping on the flat plate by primary carrier flat liquid membrane, placing in water bath for 40 seconds, and airing primary liquid film on flat plate carrier at 20-40 degrees C.