The invention discloses a method for extracting betulin from birch barks, comprising the following steps: firstly, crushed birch barks are added into an ethanol water solution, undergo reflux extraction under auxiliary processing of ultrasonic waves, and then are filtrated for removal of residue, and an extract is obtained; secondly, solid powder is separated out after decompression and concentration of the extract, and then dried to the constant weight so as to obtain betulin coarse products; and thirdly, the betulin coarse products are added into an organic solvent, undergo ultrasonic treatment and then stand, crystals are separated out, and the betulin is obtained after the separated crystals are dried to the constant weight. The method has a simple operation flow, short extraction time, low production cost and high safety, and provides a better method and a better platform for further development and utilization of natural products.