The invention provides a method for integrally desulfurizing, denitrating and removing mercury under the ultrasonic action. Hydroxyl free radicals OH* with strong oxidizability are released when a cavitation effect is generated in a reaction solution under the ultrasonic action and subjected to the oxidization removal reaction with sulfur oxides, nitrogen oxides and Hg<0> in fume; the cleaned fume is exhausted to the atmosphere; the separated mixed acid solution is applied for producing fertilizers after being absorbed by ammonia water; and the heavy metal precipitate is extracted for the recovery. An ultrasonic generator, an ultrasonic transmitter, a cooling water system, a bubbling bed reactor, an electrostatic dust collector, a fume diffuser, a separating column and a check valve are provided. The ultrasonic transmitter is disposed in the bubbling bed reactor. The fume is introduced into a pipe, sequentially passes through the electrostatic dust collector and the check valve and is introduced into the fume diffuser from the bottom of the bubbling bed reactor. A fume outlet is positioned at the upper part of the bubbling bed reactor. A residue discharge port is formed at the bottom of the bubbling bed reactor. The inlet of the separating column is connected with the outlet of the reaction products of the bubbling bed reactor. The separating columns are respectively provided with an insoluble substance outlet, a chemical feed port and a soluble substance outlet.