Abstract: The structural-acoustic coupling characteristics,mechanisms,effect of structural-acoustic coupling on natural mode and eigenfrequency of the system are analyzed theoretically and numerically.Formulae for the eigenfrequency of the coupled system are derived.Some new conclusions are obtained.The analysis results demonstrate that the strongly coupled system has obvious closed-loop feedback characteristics,whereas the weakly coupled system has obvious feedforward characteristics,and it is because of the presence of the feedback loop that the natural characteristics and eigenfrequency are changed.Cluster coupling characteristics between the structural and acoustic modes for the regular cavity and panel system are found,which determine the coupling interaction between the flexible panel and cavity.Any mode in one mode cluster only affects the modes and the modal eigenfrequencies in the same cluster independently.The modal cluster coupling changes not only the eigenfrequencies of the system but also the modal order and structural mode shape.