In this paper, experimental researches focus on the load side model and the supply side model of DC bias. On this basis, considering the transformer magnetic leakage and hysteresis effect, an improved circuit-magnetic coupling model is proposed to accurately reflect the distorted situation of bias current. Then the load excitation characteristics of a single-phase transformer under DC bias were calculated and analyzed. Finally the experimental results prove the validity of the proposed method and provide methods and ideas to study the excitation characteristics of the transformer