A Legendre polynomial approach (PA) is proposed to forecast a thin film of transverse-type rectangular piezoelectric transformer (PT) performance with common ground electrodes located on its whole bottom surface, fabricated from a PZT5A (lead zirconate titanate) ceramic material. Based on the automatic incorporation of boundary conditions into the equations of motion and the development on a basis of orthonormal polynomials of the fields, formulations calculated analytically are simulated numerically. Then, series and parallel resonance frequencies, profiles of the mechanical displacement and the electrical potential are obtained for the PT. Our results are validated through a comparison with the three-dimensional Finite Element Method (FEM) ones. Furthermore, the mechanical tethers positions are located at the PT’s junctions.