The flameproof thermoplastic resin composition of the present invention comprises (A) 40 to 95 parts by weight of a rubber modified styrene-containing resin composed of (a1) 20 to 100% by weight of a styrene-containing graft copolymer resin including 18 to 50% by weight of acrylonitrile in the copolymer excluding rubber; and (a2) 0 to 80% by weight of a styrene-containing copolymer resin containing 18 to 50% by weight of acrylonitrile; (B) 5 to 60 parts by weight of polyphenylene ether resin; (C) 2 to 40 parts by weight of a styrene-containing copolymer or styrene-containing graft copolymer resin containing 5 to 18% by weight of acrylonitrile per 100 parts by weight of the sum of (A) and (B); and (D) 5 to 30 parts by weight of an aromatic phosphoric acid ester compound per 100 parts by weight of the sum of (A) and (B). The method of preparing the flameproof thermoplastic resin is (i) preparing master batch in the range of 40 to 95 parts by weight of polyphenylene ether (PPE) (B), 5 to 60 parts by weight of styrene-containing graft copolymer resin or styrene-containing copolymer resin having 5 to 18% by weight of acrylonitrile content and 0 to 30 parts by weight of aromatic phosphoric acid ester compound (D) at 250 to 300° C. of molding temperature; and (ii) adding a rubber modified styrene-containing resin (A) and an aromatic phosphoric acid ester compound (D) to the master batch and extruding at 200 to 260° C. of molding temperature to be a final resin comprising 40 to 95 parts by weight of ABS resin, 5 to 60 parts by weight of PPE (B) and 5 to 30 parts by weight of aromatic phosphoric acid ester compound (D).