Abstract: For the application requirements of high integration and high selectivity of balanced filters, this paper proposes a novel balanced tri-band filter with high selectivity, which is based on an improved asymmetric stepped impedance resonator structure with self-coupling. Firstly, through the differential mode and common mode equivalent circuits of the balanced filter, the resonant characteristics of the resonator structure are specifically analyzed, and the first three resonance modes under the differential mode equivalent circuit are used to form three passbands respectively. In addition, by loading capacitors and resistance elements on the symmetrical surface of the circuit, the suppression of CM signals can be improved without affecting DM signals based on the proposed multimode balanced filter structure and design method, and a balanced tri-band filter with passband frequencies of 2.75/4.46/6.21 GHz was designed, processed and tested. The results show that the structure can achieve a compact size and high selection characteristics, and has good common mode rejection characteristics.