To accurately evaluate the energy-saving performance of a pre-swirl duct (PSD) and reveal its underlying mechanisms, this study takes an 8,800 TEU container ship as the research subject. Numerical simulations of ship model resistance, propeller open-water performance, and self-propulsion performance were performed using the Reynolds-averaged Navier-Stokes (RANS) method. The numerical method was validated against towing tank test results. Based on this, the primary energy-saving mechanisms were systematically revealed by analyzing the effects of the PSD on the wake field, propeller performance, and wake kinetic energy. At the design speed, the predicted energy-saving rates are 2.78% (CFD) and 2.74% (EFD). Under off-design conditions, the average rates are 2.86% (CFD) and 2.72% (EFD), showing good agreement between predictions and experiments. The PSD generates a pre-swirl flow via its guide fins, which optimizes the propeller inflow, improves blade performance, and suppresses transverse flow in the wake. The accurate numerical method and the systematic analysis of energy-saving mechanisms presented in this study provide a foundation for future optimization of PSDs.