Abstract: The submarine can withstand the hydrostatic pressure of several hundred meters under the deep water working environment, and its poor pressure resistance of the soft elastic medium used as the matrix of the anechoic coating limits its practical application. To solve this problem, a trapezoidal support plate is embedded in the anechoic coating of soft elastic medium with cylindrical cavities to improve the sound absorption performance under hydrostatic pressure. Based on the finite element method, the steady-state calculation and the acoustic-solid coupling model of the anechoic coating under hydrostatic pressure are established. The shape variable and stress distribution in the model are investigated, which explains the mechanism of the trapezoidal support plate to enhance the pressure resistance of the anechoic coating. The effect of gradient variation of trapezoidal support plate on the sound absorption performance under hydrostatic pressure is studied. The results show that the trapezoidal support plate with negative gradient slope can ensure the sound absorption performance of the anechoic coating more effectively under hydrostatic pressure. The influence of the width parameters of the top and bottom surfaces of the trapezoidal support plate on the sound absorption under hydrostatic pressure is studied. Results show that the trapezoidal support plate whose width of the upper surface is larger than that of lower surface can ensure the sound absorption more effectively under hydrostatic pressure. An experimental sample containing cylindrical cavity in a soft elastic medium with trapezoidal support plate was prepared. The acoustic performance of the experimental sample was measured by the pulse tube method under hydrostatic pressure. The measurement results show that the sound absorption performance of the embedded support plate under hydrostatic pressure is much better than that without the trapezoidal support plate, and the embedded inverted trapezoidal support plate performs better in a low frequency range.