The invention relates to a synthetic method of flibaserin intermediate 1-(2-haloethyl)-1H-benzimidazole-2-one. The method comprises the following steps: (1) synthesis of N-monosubstituted benzimidazolone: adding N-phenyl hydroxylamine and benzaldehyde into a solvent A, stirring the mixture at room temperature overnight, then adding the mixture into a mixed liquid of iodosobenzene diacetate, trimethylsilyl cyanide and Lewis acid in an argon atmosphere, adjusting the pH value and performing extraction, drying, concentration at a reduced pressure and column chromatography; (2) synthesis of 2-haloethyl benzimidazolone: adding the N-monosubstituted benzimidazolone and 1,2-dihalothane into DMF or acetone, then adding the mixture into alkali and stirring the mixture overnight, cooling the mixture to room temperature and performing extraction, drying, concentration at a reduced pressure and column chromatography; and (3) synthesis of 1-(2-haloethyl)-1H-benzimidazole-2-one: adding 2-haloethyl benzimidazolone and concentrated hydrochloric acid into a solvent B to react, adjusting the pH value, and then performing extraction, drying, concentration at a reduced pressure and column chromatography. According to the method provided by the invention, the raw materials are low in price and easily available, and the method is convenient to operate, few in byproduct and relatively high in yield.