AbstractTo address the challenging task of effective sound absorption in the low and broad frequency band for underwater structures, we propose a novel grating‐like anechoic layer by filling rubber blocks and an air backing layer into metallic grating. The metallic gratings are incorporated into the anechoic layer as a skeleton for enhanced viscoelastic dissipation by promoting shear deformation between rubber and metal plates. The introduction of an air backing layer releases the bottom constraint of the rubber, thus intensifying its deformation under acoustic excitation. Based on the homogenization method and the transfer matrix method, a theoretical model is developed to evaluate the sound absorption performance of the proposed anechoic layer, which is validated against finite element simulation results. It is demonstrated that a sound absorption coefficient of the grating‐like anechoic layer of 0.8 can be achieved in the frequency range of 1294–10 000 Hz. Given the importance of sound absorption at varying frequencies, the weighted average method is subsequently used to comprehensively evaluate the performance of the anechoic layer. Then, with structural density taken into consideration, an integrated index is proposed to further evaluate the acoustic properties of the proposed anechoic layer. Finally, the backing conditions and the boundary conditions of finite‐size structures are discussed. The results provide helpful theoretical guidance for designing novel acoustic metamaterials with broadband low‐frequency underwater sound absorption.