In order to study the core loss of transformers under DC Bias, A core loss simulation model of a 500kV nuclear power transformer based on a A– φ method of two finite element transient magnetic under DC Bias has been proposed in this paper. In this model, add the DC Bias current from 0A to 30A, the distribution of core loss in different core routes of transformers can be presented. And from the results, they show with the increase of the DC Bias current, the excitation current produces distortion and large harmonics, and the core loss increase, the distribution of core loss changes. Among them, the area connected the main yoke with the side yoke are more influenced ,and the horizontal routes are more influenced by the DC Bias than the vertical routes.