A flow-electrode capacitive deionization device, comprising: a flow path through which an electrolyte which is fed flows; a cation exchange membrane disposed at one side of the flow path; an anion exchange membrane disposed at the other side of the flow path; a cathode collector located outside the cation exchange membrane with regard to the flow path; an anode collector located outside the anion exchange membrane with regard to the flow path; and flow electrodes flowing between the cation exchange membrane and the cathode collector and between the anion exchange membrane and the anode collector, wherein the cation and anion exchange membranes comprise a porous polymer substrate and a polymer electrolyte incorporated into pores of the porous polymer substrate, wherein the polymer electrolyte in the cation exchange membrane is formed by incorporating (i) a solution including a crosslinkable aromatic polymer represented by Chemical Formula 1 below, a monomer having an aliphatic or aromatic ring with a vinyl group linked thereto preferably selected from a vinylpyrrolidone monomer, a styrene monomer or a divinylbenzene monomer, and an initiator into the porous polymer substrate and then performing crosslinking polymerization: wherein W, X and Y are each independently one or more selected from among SO 2, CO 2, CH 3 -CH 2 -CH 3, CF 3 -CF 2 -CF 3 and S, Z is any one selected from among C-CH 3, C-CF 3 and C-H, and a + c = b + d, or (ii) a solution including a crosslinkable aromatic polymer represented by Chemical Formula 2 below into the porous polymer substrate and then performing crosslinking polymerization: wherein x and y are greater than 0, x + y = 1, and n is an integer of 1 or more, or (iii) a solution including a styrene-based monomer preferably selected from one or more from the group consisting of alpha-methylstyrene, styrene and trifluorostyrene, vinylpyrrolidone, a crosslinking agent preferably selected from divinylbenzene, methylenebisacrylamide or tetramethylenebisacrylamide and an initiator preferably selected from N,N'-azobisisobutyronitrile (AIBN), benzoyl peroxide or cumyl peroxide, into the porous polymer substrate and then performing crosslinking polymerization. a flow path through which an electrolyte which is fed flows; a cation exchange membrane disposed at one side of the flow path; an anion exchange membrane disposed at the other side of the flow path; a cathode collector located outside the cation exchange membrane with regard to the flow path; an anode collector located outside the anion exchange membrane with regard to the flow path; and flow electrodes flowing between the cation exchange membrane and the cathode collector and between the anion exchange membrane and the anode collector, wherein the cation and anion exchange membranes comprise a porous polymer substrate and a polymer electrolyte incorporated into pores of the porous polymer substrate, wherein the polymer electrolyte in the cation exchange membrane is formed by incorporating (i) a solution including a crosslinkable aromatic polymer represented by Chemical Formula 1 below, a monomer having an aliphatic or aromatic ring with a vinyl group linked thereto preferably selected from a vinylpyrrolidone monomer, a styrene monomer or a divinylbenzene monomer, and an initiator into the porous polymer substrate and then performing crosslinking polymerization: wherein W, X and Y are each independently one or more selected from among SO 2, CO 2, CH 3 -CH 2 -CH 3, CF 3 -CF 2 -CF 3 and S, Z is any one selected from among C-CH 3, C-CF 3 and C-H, and a + c = b + d, or (ii) a solution including a crosslinkable aromatic polymer represented by Chemical Formula 2 below into the porous polymer substrate and then performing crosslinking polymerization: wherein x and y are greater than 0, x + y = 1, and n is an integer of 1 or more, or (iii) a solution including a styrene-based monomer preferably selected from one or more from the group consisting of alpha-methylstyrene, styrene and trifluorostyrene, vinylpyrrolidone, a crosslinking agent preferably selected from divinylbenzene, methylenebisacrylamide or tetramethylenebisacrylamide and an initiator preferably selected from N,N'-azobisisobutyronitrile (AIBN), benzoyl peroxide or cumyl peroxide, into the porous polymer substrate and then performing crosslinking polymerization. (i) a solution including a crosslinkable aromatic polymer represented by Chemical Formula 1 below, a monomer having an aliphatic or aromatic ring with a vinyl group linked thereto preferably selected from a vinylpyrrolidone monomer, a styrene monomer or a divinylbenzene monomer, and an initiator into the porous polymer substrate and then performing crosslinking polymerization: wherein W, X and Y are each independently one or more selected from among SO 2, CO 2, CH 3 -CH 2 -CH 3, CF 3 -CF 2 -CF 3 and S, Z is any one selected from among C-CH 3, C-CF 3 and C-H, and a + c = b + d, or (ii) a solution including a crosslinkable aromatic polymer represented by Chemical Formula 2 below into the porous polymer substrate and then performing crosslinking polymerization: wherein x and y are greater than 0, x + y = 1, and n is an integer of 1 or more, or (iii) a solution including a styrene-based monomer preferably selected from one or more from the group consisting of alpha-methylstyrene, styrene and trifluorostyrene, vinylpyrrolidone, a crosslinking agent preferably selected from divinylbenzene, methylenebisacrylamide or tetramethylenebisacrylamide and an initiator preferably selected from N,N'-azobisisobutyronitrile (AIBN), benzoyl peroxide or cumyl peroxide, into the porous polymer substrate and then performing crosslinking polymerization.