This paper presents the optimal design of axial-based piezoelectric generators. The optimization problem is solved for the combination of the two different types of poling piezo elements used and the total cumulative voltage output. The task is to optimize the design in such a way as to obtain the optimal output voltage for a given mechanical excitation. The Piezoelectric Energy Generators (PEGs) have two domains—active and passive domains. The optimization process is divided into several steps, which significantly reduces the number of calculations. This paper is focused on the optimization process in the passive domain, that increases the output voltage for given mechanical excitation. In the optimization, the process can be developed for specific operating conditions, various lengths of duralumin base plate, the various position of proof mass, and different applied acceleration. It has been modeled and analyzed for axial-based piezoelectric generators. The maximum voltage and power are observed 11.64 V and 1355 µw at 633 Hz, respectively, when the length of the duralumin base plate is 150 mm and 5 m s−2 acceleration. The analysis of this study can guide the passive domain optimization of PEGs to meet desired purposes of energy harvesting.