Based on the time-domain field-circuit coupling model of UHV transformer, the partial differential equation model of transient circuit is established by using the principle of energy disturbance in the magnetic field model and the dynamic inductance parameters in the circuit model. For three different load types, resistance, induction and capacitance, the influence of DC bias on winding currents and their harmonics of UHV transformer are calculated. Facing the extremely long transition process caused by large inductance and small resistance of UHV transformer and the inundation of the small DC voltage relative to the 1000 kV AC voltage during the iterative calculation, the time to steady state is greatly shortened by adding series resistance in circuit model, and the calculation error caused by series resistance value is effectively eliminated by voltage iteration compensation. The correctness of the model is verified by comparing the value of DC-bias current and the value of DC component of the series winding current.