This article studies the structure–acoustic coupling mechanism between two adjacent flexible panels and an enclosed cavity by analytical and mathematical methods based on modal expansion methods and impedance mobility techniques. The results show that the coupling relations among subsystem modes of the coupled system have selectivity characteristics. The coupling strength depends on the normalized mode–shape coupling coefficients. The coupling relationship between two flexible panels is established through the enclosed cavity. The structural–acoustic coupling effect mainly affects the low-order modes of the coupled system, especially the first-order modes of the panels and cavity. When one panel is weakly coupled with the cavity, the two flexible panels are decoupled. The vibration of the panel only depends on its structural characteristics and external excitation, and the panel radiates sound into the cavity. The vibration of another panel depends not only on its structural characteristics but also on its coupling effect with the cavity.