The present invention relates to a poly(ethylene-aliphatic diene)-g-polystyrene-based copolymer having an improved mechanical property and heat resistance, and a method for preparing the same. More particularly, the present invention relates to a poly(ethylene-aliphatic diene)-g-polystyrene-based copolymer, which includes an ethylene-aliphatic diene copolymer as a soft segment and a polystyrene-based polymer grafted to the soft segment as a hard segment, and a method for preparing the same. The poly(ethylene-aliphatic diene)-g-polystyrene-based copolymer has elasticity equivalent to that of SBS or SEBS, and improved mechanical properties such as tensile strength and heat resistance, thereby being used as a substituent for SBS or SEBS.