The present invention relates to a method of preparing a thermoplastic resin. More particularly, the present invention relates to a method of preparing a thermoplastic resin, the method including A) a step of coagulating an emulsion-polymerized latex with an acid; B) a step of treating the coagulated aggregate (slurry) with a base, followed by dehydration; and C) a step of treating the dehydrated wet powder with an acid, followed by dehydration. A method of preparing a thermoplastic resin which exhibits reduced gas generation, color improvement, and superior thermal stability is provided.