The invention discloses a magnetic rare earth barium ferrite nano purifying catalyst, as well as a preparation method and application thereof. The catalyst is used for waste water treatment. The general formula of the catalyst is BaxReyMezFemOn, wherein x is 0.1-3.0, y is 0-2.0, z is 0-2, m is 1-24, n is 3-48, Me is Cu, Zn, Mn or Co, and Re is La, Ce, Nd, Pr, Sc or Y. The preparation method comprises the following steps: dissolving to prepare a solution, matching, mixing, carrying out high temperature ultrasonic dispersion for solation, carrying out microwave vacuum for gelation, carrying out microwave vacuum for denitration with less oxygen, and roasting; the method is characterized in that the high temperature ultrasonic dispersion, the microwave vacuum for gelation, the microwave vacuum for denitration with less oxygen and the roasting processes are performed after the matching and mixing processes. According to the invention, microwave vacuum for denitration with less oxygen is adopted to enable ammonium nitrate to be directly transformed into nitrogen, oxygen and water, no nitrogen oxide is generated during the preparation process, and secondary pollution caused by nitrogen oxide generated during a traditional preparation process is eliminated.