Purpose <p>This paper is focused on vibration and acoustic analysis of functionally graded porous piezoelectric (FGPP) annular plate backed by cavity, which may be often encountered in marine and aerospace structures. Nonhomogeneous material and mechanical-acoustic-electric coupling lead to challenges for the vibro-acoustic modeling and analysis.</p> Methods <p>Based on a modified variational principle, a semi-analytical method is developed for the FGPP annular plate-cavity with various boundary constrains. In order to naturally include the electric, acoustic and mechanical continuity at the boundaries and interfaces between adjacent subdomains, the Lagrange multiplier method is combined with least-square weighted residual method for both the plate and acoustic cavity. The fluid-structure coupling between the plate and cavity is incorporated by the work functions and velocity continuity at the interface. Expanding the response component analytically in the circumferential direction and numerically in the axial direction gives the semi-analytical solution and automatically identify each circumferential mode.</p> Results <p>A series of numerical results are compared with those from references and finite element (FEM) analysis to show excellent convergence, accuracy, and efficiency of the proposed method.</p> Conclusion <p>It can be found that under the boundary conditions considered in this paper, Enlargement of the cavity length will firstly weaken the stiffness coupling between plate and cavity, and then reinforce the mass coupling between fluid and structure. For the forced vibro-acoustic responses, the most efficient mode is the n = 0 mode and dominate the acoustic responses. Since the mode is quite sensitive to the cavity length, one can optimize the length to strengthen or reduce sound radiation of the coupled system.</p>