The invention discloses a synthetic method of 3, 5-dichloro-2-pentanone. Under the action of salt and a phase transfer catalyst, alpha-chloro-alpha'-acetyl-gamma-butyrolactone, which is used as a raw material, reacts with hydrochloric acid; and then, 3, 5-dichloro-2-pentanone is synthesized through a continuous steam distillation method. According to the characteristic of great boiling point difference between the raw material and the product, the technical scheme of reacting while separating is adopted. Therefore, purity of the product is raised, yield is greatly increased, and the hydrochloric acid solution in the reaction can be recycled. By adding the phase transfer catalyst, reaction speed of the raw material can be raised, and accumulation of the raw material in the reaction system is reduced. By adding salt, reaction distillation temperature can be increased, product separation speed is accelerated, and retention time of the raw material and the product in the reaction system is shortened. Thus, side reaction is minimized, and selectivity is enhanced.