Abstract: Objectives This study aims to establish a vibro-acoustic coupling model for underwater sandwich panels coupled with an acoustic cavity and backplate under elastic boundary constraints, and investigates the effects of backplate damping, backplate parameters, and acoustic cavity parameters on structural and acoustic responses under mechanical excitation. Methods First, a dynamic model for rectangular plates is developed based on thin-plate theory, while the Layerwise theory is adopted to formulate dynamic models for damped plates and sandwich panels. The modified Fourier series is employed as the displacement trial function for the plate structures, with elastic boundary constraints simulated using artificial boundary springs. Response functions for plates, damped plates, and sandwich plates under elastic boundaries are derived via energy variational principles. By coupling the sandwich panel and plate (damped plate) with a sealed acoustic cavity and a semi-infinite acoustic field, a coupled underwater sandwich panel-acoustic cavity-plate (damped plate) model adaptable to diverse structural boundary constraints and acoustic cavity conditions is established, along with a solution method for vibro-acoustic responses. Results The damped plate effectively reduces system responses in high-frequency ranges. Increasing the backplate thickness decreases the overall mean-square vibration velocity of the backplate and significantly reduces the mean-square acoustic pressure within the acoustic cavity at mid-to-low frequencies. The acoustic cavity height primarily influences the coupled structural-acoustic system before the first-order resonance peak.Conclusions The proposed underwater sandwich panel-acoustic cavity-plate coupling model demonstrates high accuracy and provides a valuable reference for the acoustic design of sonar cavities.