Abstract: A multi-cavity output system with radial coupling is provided with broadband characteristics (frequency dispersed distribution in broadband). In this paper, using the method of equivalent circuit, the equivalent circuit of the radial coupling multi-cavity res-onator is presented. After determining the nature of radial coupling holes, applying Kirch-hoffs second law to the circuit and resulting the equation in matrix form, the three resonance frequencies are given by solving impedance matrix obtained. One of three resonance frequen-cies, which is in middle of three is selected to obtain the mode structure and the ratio of gaps amplitude of equivalent voltage of main cavity to vice-cavity. One typical mode distribution is 2ir-ir-2ir in the radial coupling cavities, it is different from part in axial coupling cavities (two-gap extended-interaction resonantors).