This paper studies the dynamics of a transient-motion piezoelectric energy harvester (TM-PEH) with a piezoelectric beam and two magnets. We first established the governing equations based on dynamic theories and the dipole–dipole model. Subsequently, an equivalent circuit model (ECM) of the TM-PEH is built based on electromechanical analogies to analyze the effects of various system parameters on the performance of this TM-PEH. Particularly, a decoupling phenomenon is observed when the mover’s speed exceeds a threshold, referred to as the decoupling velocity. Once the speed rises over it, the efficiency of the TM-PEH will decrease. Moreover, we utilized a self-powered synchronous electric charge extraction (SP-SECE) circuit to further improve the efficiency of the TM-PEH. Our analysis revealed that with the increase of the mover speed, using the SP-SECE can help harness more energy during the plucking motion. However, if the mover speed is below a threshold, the SP-SECE circuit will deteriorate the performance of the TM-PEH. The above results indicate that any TM-PEH design has an optimal applicable speed range. Following the analysis procedures presented in this paper, one can accurately identify and optimize the optimal speed range of a given TM-PEH design.