The coupled system model between stiffened plate and trapezoidal cavity is estimated using modal expansion method and vibro-acoustic coupling theory. Based on the coupled model，the effect of four different factors on response of coupled system is conducted，including distance between rib and excitation source，stiffness and mass of stiffened rib，the number of ribs. The results show that the response of ribbed plate and trapezoidal cavity is closely related to the distance between the point force and rib compared with the case of uniform plate. When the point force is directly acted on the rib or the distance between the rib and point force is closed，the reduction of plate vibration energy and acoustic potential energy appear among many frequency bands. However，there will be no reduction phenomenon when the distance between the point force and rib is large. The reduction of acoustic potential energy inside trapezoidal cavity and clamped plate vibration energy become larger as the bending stiffness of beam increases.The mass of beam has different influences on the variation of energy with different frequency bands. As the number of ribs increases，the energy stored in the trapezoidal cavity and clamped plate cannot always appear reduction phenomenon.