NOVELTY - Synthesizing finasteride by chlorination and dehydrogenation, involves: using dihydro-finasteride as the raw material, performing chlorination reaction to prepare a 2-chlorodihydrofinasteride intermediate under mild conditions, and then removing hydrogen chloride by an elimination reaction of the 2- chlorodihydrofinasteride so as to obtain the finasteride.    USE - For synthesizing finasteride by chlorination and dehydrogenation (claimed), which is useful for treating prostate disease.    ADVANTAGE - The synthesis method of finasteride provides cheap raw materials, mild reaction conditions, fewer side effects, fast response, simple product isolation and high yield of finasteride. The present method develops simple reaction, high yield, easy separation, high safety, low cost and environmentally friendly production technology for improving environment.    DETAILED DESCRIPTION - Synthesizing finasteride by chlorination and dehydrogenation, involves: synthesis of a 2-chlorodihydrofinasteride intermediate by: adding dihydro-finasteride to an organic solvent, slowly adding (1,8-diaza-bicyclo(5,4,0)undec-7-ene) (DBU) under protection of nitrogen, keeping the temperature at -5 to 10 degrees C during adding, then adding trimethylchlorosilane in 1-2 hours, stirring and reacting the mixture for 2-4 hours at -5 to 50 degrees C after finishing, adding anhydrous lithium chloride once, increasing the temperature to the refluxing temperature, completely reacting the mixture at this temperature, decompressing and distilling out the solvent, adding a certain amount of ethanol to the residues, fully stirring and dissolving the residues, cooling the solution, then slowly pouring the solution into water for separating out the product, filtering and cleaning the separated-out product to obtain a coarse product of 2-chlorodihydrofinasteride, and decompressing and drying the coarse product at 40-70 degrees C, where the organic solvent in the reaction is one of 1,2-dichloroethane, dichloromethane, ether, tetrahydrofuran, 1,4-dioxane, toluene, benzene, xylene, carbon tetrachloride, and dimethylformamide (DMF) and the alkali is one of DBU, lithium diisopropylamide (LDA), n-butyl lithium, triethylamine, tetramethylethylenediamine, and pyridine, and the reagent is trimethylchlorosilane; and synthesis of finasteride by: adding the dried product in the first step to a reaction flask under protection of nitrogen, adding a solvent for dissolving the product, then reducing the temperature to -20 to 0 degrees C, slowly dripping alkali into the reaction flask, controlling the adding speed of the alkali, keeping the reaction temperature at -20 to 10 degrees C, slowly increasing the temperature to the room temperature after dripping the alkali, then stirring the solution for 1-2 hours, neutralizing the excessive alkali, and then cleaning, separating, concentrating and filtering the solution to obtain a coarse product of finasteride, decompressing and drying the coarse product at 40-70 degrees C to obtain a white powdery solid, and refining the product by an isopropyl acetate pulping method, where the organic solvent is one of methanol, ethanol, acetone, ethylene glycol dimethyl ether, 1,2-dichloroethane, dichloromethane, chloroform, ether, tetrahydrofuran, 1,4-dioxane, toluene, benzene, xylene, carbon tetrachloride, and dimethylformamide; and the alkali is one of K2CO3, Na3PO4, sodium acetate, potassium acetate, sodium methoxide, sodium ethoxide, potassium tert-butoxide, DBU, triethylamine, tetramethylethylenediamine, and pyridine.