The present invention relates to a styrenic thermoplastic resin composition, and more particularly to a styrenic thermoplastic resin composition comprising a non-halogen flame-retardant having superior flame-retarding properties and impact strength. The present invention provides a flame-retarding styrenic thermoplastic resin composition comprising a) 100 weight parts of a base resin comprising i) 10 to 50 weight parts of a rubber-modified styrenic-containing graft copolymer, ii) 5 to 20 weight parts of phenol novolac resin, and iii) 20 to 70 weight parts of a styrene-containing copolymer; and b) 5 to 20 weight parts of an aromatic phosphate. In addition, the a) base resin may further comprise iv) 10 to 40 weight parts of a thermoplastic polyurethane resin.