To elucidate the pressure drop characteristics and underlying mechanisms induced by a rod centralizer in a progressing cavity pump (PCP) wellbore annulus, a CFD-based multiphase flow numerical model of the wellbore annulus with a rod centralizer was developed. The centralizer's flow-induced rotation was simulated by Six-DOF solver, and pressure drop was computed. Entropy production theory and the Liutex criterion were applied to characterize energy dissipation and vortex distributions. Results indicate that, within the operating range, inlet velocity is the dominant factor governing pressure drop, with an independent contribution of 97.37%. More than 60% of the total pressure loss occurs in the centralizer annulus region. Entropy production components were quantified. The time-averaged component accounts for more than 50% of the total; with increasing inlet velocity, its fraction decreases, whereas the turbulent component fraction increases and the wall component fraction changes only marginally. Vortices were found to cluster in high entropy production regions, indicating that local vortex accumulation triggered by abrupt geometric transitions and flow-induced rotation of the centralizer is a primary cause of increased pressure loss. These findings advance the understanding of pressure drop mechanisms and flow field characteristics in PCP wellbores and provide insights for pressure drop prediction and control.