The invention discloses a method for preparing phentermine hydrochloride. The method for preparing the phentermine hydrochloride is characterized in by comprising the following steps: adding dimethyl benzyl carbinol serving as raw material into a mixed solution of acetic acid and an organic solvent, then dropwise adding sulphuric acid serving as a catalyst, stirring for reaction, then adding water until the solution is turbid, adding aqueous alkali for regulating pH to 6-8, and then carrying out reduced-pressure suction filtration to obtain N-(1,1-dimethyl-phenethyl) acetamide; then dissolving N-(1,1-dimethyl-phenethyl) acetamide into an organic solvent, carrying out heating reflux in the presence of inorganic strong alkali solid serving as an acid-binding agent, and cooling to room temperature; adding water, stirring and extracting a product; drying with an anhydrous drying agent overnight; removing the solid, carrying out reduced pressure distillation on filtrate by adopting a water pump, so as to obtain phentermine free alkali; then dissolving with the organic solvent; adding hydrochloric acid under an ice bath condition or introducing dry hydrogen chloride gas, then stirring and carrying out suction filtration to obtain the phentermine hydrochloride. The method for preparing the phentermine hydrochloride has the advantages of high yield, low cost and no adoption of column chromatography and is suitable for large-scale industrial production.