This paper proposes a method of the coupling modal, which is able to miniaturize the tunable cavity filter while keeping its bandwidth balancing. The filter consists of a tunable cavity dual-bandpass filter and a triangular twin-loop as its inter-cavities coupling structure. We analyzed and calculated the bandwidth of the filter changing with the size and position of the triangular twin-loop. To prove the advancement of the design, a tunable coaxial cavity dual-bandpass filter operating at 230MHz and 409MHz was fabricated and measured. The size is less then a half that of the conventional tunable filter with same specifications. The insertion loss is lower than 1.2dB at operating frequencies. And the bandwidth in lowband and highband are both more than 2.5MHz with the insertion loss less then 3dB. Experiment results and theoretic analysis agree well. This novel model can contribute to the miniaturization of RF and microwave systems with high quality.