Provided are a process for preparing an expandable polystyrene (EPS) resin using polyoxyethylene sorbitan monolaurate and a polyethylene wax synthesized from methylene oxide, and an EPS resin composition including a Sasol-wax which is a polyethylene wax synthesized from methylene oxide. The EPS resin is excellent in expandability and strength, and can be processed into a desirable foamed product even when it is subjected to short-term maturing after pre-expansion. Addition of divinylbenzene as a crosslinking agent at an initial stage of polymerization and addition of a mixture of butane, pentane, and cyclohexane as a blowing agent permit the EPS resin to be expanded at a high expansion ratio of 93 times or more. Further, addition of a polyethylene wax synthesized from methylene oxide at an initial stage of polymerization and addition of polyoxyethylene sorbitan monolaurate in the course of the polymerization, in particular when a polymerization ratio reaches 70-90%, can impart good strength to the EPS resin.