The converter transformer saturation model founded in the paper is based on UMEC (Unified Magnetic Equivalent Circuit) theory, which considers core flux and leakage flux uniformly, with high simulation accuracy. Based on the model founded, detailed simulations are made about the converter transformer magnetizing current, the harmonic characteristics of the primary-side and secondary-side current of the converter transformer and the reactive power characteristics by PSCAD simulation software. Finally, this paper presents some cure strategies to inhibit converter transformer DC bias.