Abstract: ［Objective］ To ensure the safety of traction transformers in dual current power supply lines and prevent issues such as transformer vibration and winding temperature increase caused by excessive DC bias current, which can lead to severe distortion of excitation current, it is essential to study the degree of DC bias in transformers of dual current power supply lines. ［Method］ The DC bias problem in transformers of dual current power supply lines is introduced. By dividing the line into smaller sections, with trains in the DC segment modeled as power sources, and trains in the AC segment and traction transformers modeled as resistances, a chain-circuit model for the dual current power supply system is established. Using continuous linear load flow calculation method, the DC bias current of transformers in the AC segment of a dual current power supply line is computed. The calculated results are compared with field-measured data to verify the validity and accuracy of the proposed method. ［Result & Conclusion］ The DC bias current in transformers significantly exceeds the standard at certain times. The proposed calculation method, characterized by minimal admittance matrix order and fast computation speed, is suitable for dynamic calculations of DC bias currents in transformers of dual current power supply lines.