TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The reducing agent is lithium aluminum hydride (LiAlH4), hydrogen (H2)/nickel (Ni), preferably sodium borohydride (NaBH4), or potassium borohydride (KBH4). The chlorinated reagent is phosphorus oxychloride, or preferably thionyl chloride.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The reaction solvent used in reduction reaction is water, methanol, isopropanol, or preferably ethanol. The reduction reaction temperature is 10-40 preferably 25-30 degrees C; and the reduction reaction time is 1-3, preferably 2.0 hours. The molar ratio of 2-amino-3,5-dibromobenzaldehyde and reducing agent is 1.5-2.5:1.0, preferably 2.2:1.0. The chlorination reaction temperature is 0-40, preferably 25-30 degrees C; and the chlorination reaction time is 10-14, preferably 12 hours. The amount of chlorinated reagent is 2.5-3.5, preferably 3.0 times the amount of 2-amino-3,5-dibromophenylcarbinol (volume mass ratio: g/ml). The amination reaction temperature is less than 65, preferably 25-65 degrees C; and the reaction time is 2.0-2.5, preferably 2 hours. The molar ratio of 2,4-bromo-6-chloromethylaniline and N-methylcyclohexylamine is 1.0:2.5-4.0, preferably 1.0:3.0. The amination reaction comprises directly adding 2,4-bromo-6-chloromethylaniline in batches to N-methylcyclohexylamine. The HCl salifying reagent is HCl gas, or preferably HCl alcohol solution, particularly HCl 1-5C lower alcohol solution i.e. HCl ethanol solution. The HCl alcohol solution is prepared by reacting alkyl acyl chloride and alcohol. The HCl ethanol solution is prepared by reacting acetylchloride and ethanol. The preparation method further comprises removing the rest chlorinated reagent after the chlorination reaction, preferably processing reduced pressure distillation operation under room temperature; decoloring with activated carbon (4-8, preferably 5 mass% of 2,4-bromo-6-chloromethylaniline) after amination reaction and before the salifying reaction; recrystallizing to bromhexine hydrochloride in organic solvent; and decoloring with activated carbon (3-5, preferably 3 mass% of the mass of bromhexine hydrochloride) for 20-30 minutes. The organic solvent used in recrystallization is ethanol, isopropanol, or preferably methanol. The ratio of the mass of bromhexine hydrochloride to the volume of organic solvent is 1:12-20, preferably 1:15 (g/ml) in the recrystallization step. In the step of recrystallization, the crystallization method is natural cooling and the crystallization time is 5-8, preferably 6 hours.