The invention discloses a method for synthesizing hydromorphone by using a continuous flow microchannel reactor. According to the method, by the temperature and the reaction speed of each module of the continuous flow microchannel reactor are controlled, a quenching is integrated into the continuous flow microchannel reactor, and by control of optimal reaction state of hydromorphone synthesis rawmaterials and a catalyst and the reaction speed, the reaction speed is improved. At the same time, through addition of the quenching module, the reaction can be promptly quenched, the precise controlof the reaction time can be achieved, over oxidation in a key step of Oppenauer Oxidation during hydromorphone synthesis can be reduced, production of by-products can be reduced, and reaction yield and hydromorphone product purity can be improved.