The improved time-stepping finite element method (TSFEM) is coupled with circuit equations to study the DC-biased phenomenon of transformer in this paper. The periodic variation conditions of excitation current and magnetic field are considered, and the locally convergent fixed-point method is used to solve the nonlinear time-varying equation. Besides, a fast solution called series resistance and voltage compensation (SRVC) method is utilized to shorten the transient process and solve the stability problem caused by the small resistance and the drastic change of inductance in the DC-biased calculation of transformers. The method presented in this paper is verified on a square laminated core under DC bias. And some problems brought by transformer DC bias and some measures in term of transformer itself to suppress the transformer DC bias are discussed.