Based on the transient“field-circuit coupled" method，two-dimensional solution mode is establishedaccording to the parameters of the actual product，so that people can further research the effect on the operatingperformance of converter transformer caused by DC bias better. Considering the nonlinearity of core material’s permeability，the model is used to analyze the exciting current waveform curves and the iron core magnetic flux intensity changes of no-load in different DC bias conditions. In addition，the condition of no-load loss is analyzed quantificationally in accordance with half-wave average algorithm. The results show that the increase of DC bias ratio leadsto distortion of the exciting current waveform. The low harmonics become more and more abundant. The peak valueof iron core magnetic flux intensity grows nonlinearly and tends to be saturated. Synchronously the no-load loss isalso increased.This method provides an effective means for the design and maintenance of converter transformer’ sDC bias.