The present invention relates to an acrylate-styrene-acrylonitrile polymer, a method for preparing the same, and a thermoplastic resin composition. More specifically, the present invention has an effect of providing an acrylate-styrene-acrylonitrile polymer, a method for preparing the same, and a thermoplastic resin composition, wherein an N-vinyl monomer is applied such that the polymerization can be initiated by a redox system under conditions in which a grafting agent is not used, thereby improving the grafting efficiency of the acrylate-styrene-acrylonitrile polymer and thus improving powder characteristics and impact resistance while maintaining dye-ability; the content of free shells is lowered to improve the coagulation and latex stability; the polymerization is performed by the redox system without an activation liquid of a shell layer, thereby reducing additives and thus improving pigment dye-ability and significantly improving weather resistance.