The invention discloses a method for continuously synthesizing cumarin by a microchannel reactor, comprising the following steps: (1) dissolving a catalyst into a mixed solution of acetic anhydride and acetic acid, wherein the mass ratio of the acetic anhydride to the acetic acid to a catalyst is (15-50): (2-5):1; (2) respectively pumping salicylaldehyde and the mixture obtained in step (1) in the volume ratio of 1: (1-2) into the microchannel reactor, and enabling the mixture to pass through a reactor I and a reactor II connected in series, wherein the total dwell time of the mixture in the reactor I and the reactor II lasts for 20-80 min; and (3) refining the discharged material to obtain a cumarin finished product. By using the method, tower and kettle devices are avoided, and the method is short in reaction time, low in raw material consumption per unit, high in conversion rate and yield, low in cost and suitable for industrial application.