Flame retardant resin composition and method for preparing the same

A method for preparing a flame retardant resin composition, the method comprising:

• mixing and extruding a graft copolymer comprising a conjugated diene-based polymer, a first aromatic vinyl-based monomer unit, and a first vinyl cyan-based monomer unit;
• a matrix copolymer comprising a second aromatic vinyl-based monomer unit and a second vinyl cyan-based monomer unit;
• a polyester-based resin;
• a polymeric phosphorus-based flame retardant;
• and a first flame retardant aid comprising an (alkyl)methyl acrylate copolymer,
• wherein the polymeric phosphorus-based flame retardant includes at least one structural unit represented by Formula 1 below:
• wherein in Formula 1, R1 to R3 are each either a hydrogen or an alkyl group having 1 to 30 carbon atoms,
• A1 and A2 are each an arylene group having 6 to 20 carbon atoms, and
• n is a natural number.