Abstract: Based on the principle of the transfer function method, a numerical analysis model for the underwater acoustic characteristics of composite sandwich materials with cavities and grid reinforcements was established using the finite element method, while accounting for the frequency-dependent properties of the materials. The accuracy of the computational method was verified by comparing it with theoretical solutions and experimental results. Subsequently, a systematic study was conducted on the influence of cavity volume and shape on the absorption properties and mechanisms of such complex-configuration acoustic materials. Furthermore, the sound absorption characteristics of a novel multi-layer composite structure were designed and analyzed. The research results indicate that larger cavities lead to better sound absorption performance below 6000 Hz. In the absence of resonance, the primary mechanism involves reducing structural stiffness and increasing the vibrational strain at the coupling interface between the sound-absorbing material and the cavity, thereby dissipating acoustic energy. Spherical and conical cavities exhibit an impedance gradient effect under incident sound waves, contributing to the first two absorption peaks. However, the presence of a steel spherical shell shields the cavity's resonance mechanism, preventing improvement in absorption below 6000 Hz. On the other hand, under the dual mechanisms of impedance mismatch between the thin shell and the sound-absorbing material and elastic resonance, the material's absorption performance is enhanced above 8000 Hz, with greater improvement observed as the cavity volume increases. This study provides theoretical and technical support for the application of such materials in underwater engineering structures and equipment.