Abstract: The experiments and the simulations are performed to analyze the full spectrum acoustic response of a composite thin plate structure under a diffusive sound field (SPL=140 dB). In a typical reverberation chamber, the diffusive sound field and the structural acceleration responses are measured for the C/SiC composite structure. A hybrid FE-SEA model is established for simulating the interactions of the orthotropic composite thin plate structure with the acoustic load, and the simulation results are in agreement with the experimental tests. Meanwhile, the stress field inside the composite structure is obtained by the modal stress recovery method. It is shown that the distribution of the stress field is different from that of the acceleration field for the composite structure under the diffusive sound field. The relation between the structural bending wavelength and the acoustic wavelength is analyzed to reveal the coupling mechanism between the acoustic load and the composite structure. The stress field obtained by the hybrid FE-SEA model can be further applied to the acoustic fatigue analysis.