The invention discloses a method for preparing micron-level triangular cone-shaped cuprous bromide by using an ultrasonic assisted microchannel reaction device. The method includes a step (1) of dissolving divalent copper salt and bromide in water to obtain a mixed solution A; a step (2) of dissolving a reducing agent in water to obtain a reducing solution B; a step (3) of pumping the mixed solution A and the reducing solution B into a micromixer in the microchannel reaction device respectively simultaneously; after full mixing, leading the mixed solution A and the reducing solution B into a microchannel reactor in the microchannel reaction device, and performing reduction reaction under the aid of ultrasound; and a step (4) of performing centrifugation, washing and vacuum drying on a material discharged out of the microchannel reactor, to obtain the micron-level triangular cone-shaped cuprous bromide. Particles of different sizes can be prepared by changing standing time of the reaction materials in the microchannel reaction device and the size of a reaction pipe, and the method is suitable for industrially producing the micron-level triangular cone-shaped cuprous bromide with uniform particle size distribution, good dispersibility and high catalytic activity and has the advantages of simple process, continuous preparation and short reaction time.