Disclosed by the present invention is a cavity-based double-frequency filtering balun, comprising a first cavity resonator and a second cavity resonator which are connected by means of a metal plate, wherein the first cavity resonator is provided with an input end PCB; an input end slotline is provided on a metal ground layer of the input end PCB, and an input end micro-stripline is provided on the other side of the input end PCB; an input slotted hole, corresponding to the position of the input end slotline, is formed in the first cavity resonator; output end PCBs are provided on two opposite outer side surfaces of the second cavity resonator which are adjacent to the metal plate respectively; output end slotlines are provided on the metal ground layers of the two output end PCBs respectively; output end micro-strip lines are arranged on the other sides of the two output end PCBs; and output slotted holes, which correspond to the positions of the output end slotlines and communicate therewith, are formed in the second cavity resonator. The main body of the described double-frequency filtering balun comprises two cavity resonators, and such structure has high quality factors and low insertion loss. Meanwhile, the requirements of a double-pass band are met by utilizing two base membranes of the cavity resonators, so that the size of the circuit is reduced.