<p>Applying initial stress to piezoelectric elements is an important way for accurately controlling their dynamic characteristics. Based on the theories of nonlocal integral elasticity and incremental deformation, a mathematical model is established for the plane wave propagation in piezoelectric nanoplates with initial stress immersed in liquid and is solved by a presented Legendre polynomial series approach (LPSA). The mechanical and electrical boundary conditions are archived by using the Legendre polynomials. Then, the effects of nonlocal elasticity and initial stress on wave reflection, transmission, and physical field distributions are studied. Results indicate that the coupling effects on resonance frequency are mutually reinforcing, but their effects on critical angle are mutually suppressed. The study plays an important role in regulating the wave field distribution and resonant frequency in piezoelectric elements through initial stress and nonlocal effect, which is helpful for the design and optimization of micro/nano acoustic devices.</p>