Abstract: The natural frequencies and vibration modes of the bolt fixed sandwich composite structure with stiffeners were obtained using finite element method in while the surface layers were simulated as layerwise elements and the core layers were simulated as solid elements. The simulated results were verified through comparing with experimental results. The method of simulating boundary condition for the actual structure in finite element analysis is obtained. By comparing with the experimental results of steel structure, it is found that the first three-order natural frequencies are twice higher for the sandwich composite reinforced structure. The sandwich composite structure with stiffeners can greatly improve the stiffness of the whole structure and the vibration characteristics of the structure.