The invention discloses a method for preparing activated carbon by using ultrasonic. The method includes the following steps: with sawdust as a raw material, crushing, sieving through a sieve of 20-40 meshes, and baking to absolutely dry, and performing an ultrasonic treatment by using ultrasonic to prepare sawdust powder; then, mixing and dipping the sawdust powder with a phosphoric acid solution for 3-5 hours, activating, cooling to room temperature and cleaning to prepare an activated material; drying, crushing and sieving the activated material to prepare activated carbon. The method provided by the invention is simple to operate and is easy to implement. The sawdust used in the invention is a wooden raw material containing cellulose, due to the ultrasonic treatment, the degree of crystallinity of cellulose is reduced and the dipping efficiency of the phosphoric acid is effectively improved, and the adsorption efficiency of the activated carbon can be improved under the same technical conditions without increasing agents, thereby reducing the manufacturing cost of the activated carbon and the pollution generated in the production process.