This disclosure relates to organic synthesis, and more particularly to a method for preparing 3-chloro-4-oxopentyl acetate using a fully continuous-flow micro-reaction system. In this method, chlorine and an acetylbutyrolactone-containing liquid are simultaneously transported to a first micro-channel reactor for continuous chlorination to obtain α-acetyl-α-chloro-γ-butyrolactone. The reaction mixture is simultaneously transported to a micro-mixer and a second micro-channel reactor together with a mixed solution of glacial acetic acid, hydrochloric acid and water, and the continuous acylation is carried out to obtain 3-chloro-4-oxopentyl acetate. After quenched with a quenching agent, the reaction mixture was subjected to extraction and separation to obtain the 3-chloro-4-oxopentyl acetate.