Due to the coupled mechanical and electrical properties, piezoelectric materials are widely adopted for sensing purpose. In order to predict the device behavior in the design phase, many finite element tools were developed. However, most of the elements did not concern about the equipotential nature of sensor electrodes and the equipotential constraint has to be imposed in the structure level. This paper develops a composite plate element that the equipotential condition of electrode is ensured automatically. The element is displacement-electric potential type that can model elastic plates bonded with piezoelectric sensing layer. The formulations of displacement and electric potential fields are based on Mindlin plate model and the element is deduced from Hamilton's principle. In order to model physical behavior reasonably, different power series are assigned to displacements and electric potential respectively. Employing penalty function method imposes the equipotential condition on element electrode. Thus the element has the capability to analyze deformation, natural frequencies, and electrical signal efficiently. Patch tests are carried on different problems whose analytic solutions are available. In static constant stress situations, these tests show that the element correctly finds out the displacements, stresses, and electric potential. The excellent convergent rate of the natural frequencies demonstrates it is also good for dynamic analysis.