Abstract: Deeply analyzing the physical mechanisms of active sound insulation of the rib stiffened double-panel struc- ture can offer helps for optimally designing the system. The theoretical model of such system is established using the combined modal expansion and vibro-acoustic coupling method. Under the condition of minimizing radiated power of the radiating ribbed plate, the physical nature of active sound insulation is interpreted in detail from the point of view of modal couplings. Results obtained demonstrate that the rule of the sound energy transmission and the physical mechanisms for the ribbed plate system are all changed, and affected by the coupling effects of the rib, when compared with the analytical results obtained for the unribbed plate system. Taking into account the coupling effects of the rib, the cavity modal suppression and rearrangement mechanisms obtained in existed investigations are modified and supplemented for ribbed plate case, which gives a clear interpretation for the physical nature involved in the active rib stiffened double-panel structure.