This paper focuses on the SZ11-10000kVA/35 kV on-load tap-changing oil-immersed power transformer, analyzing its loss characteristics via the finite element method and field-circuit coupling approach. A 3D simplified model of the core and windings was established using ANSYS Electronics, with material properties and structural details optimized for simulation efficiency. Simulations were conducted under no-load and load conditions to study iron loss, copper loss, and stray loss. Results indicate that core loss, unaffected by load, oscillates steadily with the voltage period and increases with DC bias. Winding loss, dependent on load current, rises significantly (over 17% at 1000 V neutral point voltage) as the neutral point voltage increases. Stray loss accounts for a minimal proportion. This research provides theoretical and data support for optimizing transformer design and enhancing operational efficiency.