Abstract: Aiming at the problems of excessive eddy current loss and local overheating induced by leakage magnetic field on the tank wall of large-capacity rectifier transformers such as 27000 kVA under high-harmonic and high-current operating conditions, this paper conducts relevant research. The traditional 12-pulse rectifier system adopts the Y/Δ connection of the valve-side windings with a 30° phase angle difference, which easily causes the "forward superposition" of the leakage magnetic fields of the two groups of windings, enhances the magnetic field component perpendicular to the tank wall, and aggravates the eddy current loss. To address this, this paper proposes an optimization scheme of adjusting the phase angle difference of the Y/Δ windings to 150°, which utilizes the vector cancellation effect of leakage magnetic fields to weaken the synthetic leakage magnetic intensity. A 3D finite element model including low-voltage outlet copper bars, insulation layers, and the tank wall is established, and a comparative analysis of the electromagnetic characteristics under the phase angle differences of 30° and 150° is conducted. Simulation results show that the total eddy current loss decreases from 208143.82 W to 18436.87 W, with a reduction rate of over 90%. This study verifies the effectiveness of the 150° phase angle difference scheme and provides theoretical support for the electromagnetic optimization and heat dissipation design of rectifier transformers.