In the actual operation, converter transformer valve side bushings are subjected to high voltages and large current loads with large number of harmonics simultaneously. In our previous papers, using the 3‐D electromagnetic‐fluid‐thermal coupling analysis method, the 3‐D temperature distribution of a converter transformer valve side bushing was researched, and the heating and dissipation characteristics of the bushing were analyzed. However, the electric field distribution of the bushing is affected by the temperature gradient inside the bushing. Related researches about the electro‐thermal coupling field of the bushing are rare and most researches are concentrated on the single factor of voltage or temperature. In this paper, by using the electro‐thermal coupling field simulation method proposed in our previous paper, the effect of the core outline size on the electric field distribution of the bushing under a certain temperature gradient was investigated. In addition, by using the transient electric field simulation method, the electric field distribution of the bushing in one cycle subjected to an actual voltage waveform and the current in temperature rise test simultaneously was analyzed considering the effect of the Joule heating power of the core. The proposed method will be able to provide a computation basis for the structure design and optimization of converter transformer valve side bushings. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.