This study focuses on numerical investigation and optimization of one piezoelectric-based energy harvesting pavement system (PZ-EHPS), which consists of a piezoelectric layer sandwiched between two conductive asphalt layers. Novel piezoelectric element designs inside the piezoelectric layer, such as piezo ball and piezo roof, were proposed to explore the energy harvesting potential of the system. Considering the large scale of the system, finite element models were built not only to analyze the performance of PZ-EHPS specimens fabricated in the laboratory, but also to simulate the PZ-EHPS operation under real traffic conditions. Results from the laboratory based finite element models, which were verified by experimental study, determined that a thin piezoelectric layer with piezo ball elements and a low stiffness insulative filler had the significant advantage of producing up to 85 V electricity. Compared to piezo ball elements, piezo cylinder elements may fit a thick and stiff piezoelectric layer better and lead to a high voltage output up to 50 V. As a result of the field-based finite element models, the PZ-EHPS with a rigid piezoelectric layer produces more electricity than that with a flexible piezoelectric layer. For a single-vehicle scenario, if each PZ-EHPS segment length equals the vehicle wheelbase length, consecutive PZ-EHPS segments may constantly supply high electricity as the vehicle moves at a high speed over the segments. For a multiple-vehicle scenario, however, the voltage output generated by a second vehicle will be reduced if the first vehicle is still on the same PZ-EHPS segment. This study provides optimal design features of the PZ-EHPS to lead this energy harvesting system significantly closer to the field application.