A method for preparing a flame retardant modified acrylonitrile-based copolymer, comprising: (a) prepolymerizing a first composition to obtain a first prepolymer solution including a first prepolymer; (b) prepolymerizing a second composition to obtain a second prepolymer solution including a second prepolymer; and (c) mixing the first and second prepolymer solutions and subjecting a mixture of the first and second prepolymers to polymerization to obtain a flame retardant modified acrylonitrile-based copolymer solution containing a flame retardant modified acrylonitrile-based copolymer; the method being characterized in that: the first composition includes acrylonitrile, a halogen-containing vinyl monomer, at least one hydrophilic vinyl monomer, a polar solvent, and a reaction initiator, the halogen-containing vinyl monomer including vinylidene chloride; based on the total weight of vinylidene chloride, the hydrophilic vinyl monomer and acrylonitrile of the first composition, vinylidene chloride of the first composition is in an amount ranging from 10 to 40 wt% and the hydrophilic vinyl monomer is in an amount ranging from 5 to 50 wt%; the first prepolymer has a weight average molecular weight ranging from 30,000 to 40,000; the second composition includes acrylonitrile, a halogen-containing vinyl monomer, the polar solvent, and the reaction initiator, the halogen-containing vinyl monomer of the second composition including vinylidene chloride; based on the total weight of vinylidene chloride and acrylonitrile of the second composition, vinylidene chloride of the second composition is in an amount ranging from 40 to 60 wt%; and the flame retardant modified acrylonitrile-based copolymer has a weight average molecular weight ranging from 50,000 to 80,000 and contains 30 to 46 wt% of vinylidene chloride monomer unit, 1 to 5 wt% of hydrophilic vinyl monomer unit, and 49 to 69 wt% of acrylonitrile monomer unit based on the total weight of the vinylidene chloride monomer unit, the hydrophilic vinyl monomer unit and the acrylonitrile monomer unit of the flame retardant modified acrylonitrile-based copolymer. (a) prepolymerizing a first composition to obtain a first prepolymer solution including a first prepolymer; (b) prepolymerizing a second composition to obtain a second prepolymer solution including a second prepolymer; and (c) mixing the first and second prepolymer solutions and subjecting a mixture of the first and second prepolymers to polymerization to obtain a flame retardant modified acrylonitrile-based copolymer solution containing a flame retardant modified acrylonitrile-based copolymer; the first composition includes acrylonitrile, a halogen-containing vinyl monomer, at least one hydrophilic vinyl monomer, a polar solvent, and a reaction initiator, the halogen-containing vinyl monomer including vinylidene chloride; based on the total weight of vinylidene chloride, the hydrophilic vinyl monomer and acrylonitrile of the first composition, vinylidene chloride of the first composition is in an amount ranging from 10 to 40 wt% and the hydrophilic vinyl monomer is in an amount ranging from 5 to 50 wt%; the first prepolymer has a weight average molecular weight ranging from 30,000 to 40,000; the second composition includes acrylonitrile, a halogen-containing vinyl monomer, the polar solvent, and the reaction initiator, the halogen-containing vinyl monomer of the second composition including vinylidene chloride; based on the total weight of vinylidene chloride and acrylonitrile of the second composition, vinylidene chloride of the second composition is in an amount ranging from 40 to 60 wt%; and the flame retardant modified acrylonitrile-based copolymer has a weight average molecular weight ranging from 50,000 to 80,000 and contains 30 to 46 wt% of vinylidene chloride monomer unit, 1 to 5 wt% of hydrophilic vinyl monomer unit, and 49 to 69 wt% of acrylonitrile monomer unit based on the total weight of the vinylidene chloride monomer unit, the hydrophilic vinyl monomer unit and the acrylonitrile monomer unit of the flame retardant modified acrylonitrile-based copolymer.