The invention discloses a method and a device for preventing and removing scales, removing oxygen, cleaning, sterilizing and removing algal by using ultrasonic waves. In a working medium circuit and a cooling medium circuit of a steam power plant, a filter is used for filtering dirt, and an ultrasonic processor is used for preventing and removing scales, removing oxygen, cleaning, sterilizing andremoving algal from water subjected to dirt filtration. The method and the device have the advantages that: thermal power generation process and equipment are supplemented, improved and developed by applying the acoustic technology, the steam power plant is operated on line by applying the acoustic technology, and under the action of ultrasonic cavitation effect, nanoscale cavitation closing state and negative high pressure bubble bursting characteristics, the nanoscale steam cleaning function, oxygen removal function and scale removal and cleaning function are realized. Water is taken as a carrier, and the scales are removed without dead angle when the current reaches positions with scales; substances are not added into the device, resistance is not increased, and the mechanical structure and function of the device are not changed; and the body of the conventional steam power plant is not changed, and the method and the device are suitable for promotion and application.