Ultra-high voltage direct current (UHVDC) projects have undergone rapid development in recent years. However, the DC bias phenomenon persistently exists in UHVDC transmission systems. As a critical component of DC transmission projects, the DC bias current in converter transformers can lead to distortion of the excitation current, resulting in adverse effects such as increased vibration and noise. In severe cases, it threatens the safe and stable operation of the converter transformer. This paper primarily employs multi-physical field simulation using COMSOL Multiphysics for the modeling and simulation analysis of DC bias in converter transformers. A single-phase four-leg parallel connection model of a commonly used converter transformer was established. Through field-circuit coupling, the variations in excitation current and electromagnetic fields under DC bias conditions are analyzed.