Abstract: With the cylindrical cavity-type anechoic coating as the research object, an underwater composite structure anechoic coating is proposed to overcome the limitations caused by a deep low-frequency wave valley and narrow-band acoustic absorption of the cavity resonance type anechoic coating. It is characterized by (1) the distribution of the cylindrical cavities of different scales in space appearing in the form of vortex line step-rise; (2) the introduction of metal ring wall structure outside the cylindrical cavity array. Based on the finite element analyses of the displacement change and energy dissipation characteristics of the composite structure anechoic coating and its sound absorption mechanism, the sound absorption characteristics of various combination structures are studied. This structure integrates the metal wall and the cavity resonance, which effectively improves the low-frequency sound absorption performance and achieves broadband sound absorption from 660 Hz to 15 kHz under the limit of 50 mm base material thickness, with the average sound absorption coefficient above 0.94 and can maintain good sound absorption performance under 0-2.5 MPa hydrostatic pressure. The simulation and analysis results show the feasibility of using a typical composite design of a cylindrical cavity combined with a metal ring wall to raise the low-frequency wave valley, which provides a reference solution for achieving underwater broadband sound absorption.