Abstract: Calculation and verification of DC electric field is an important part of converter transformer design. There is a great difference between the experimental results of the oil-pressboard interface charge accumulation and distribution under DC voltage and the calculation results based on the traditional resistivity-capacitance model. The electric field checking method based on ideal model is no longer in force. In this paper, an extrapolation method based on the oil-pressboard interface charge characteristics was proposed, which could be directly used in the complex structure of converter transformer. Firstly, for the uniform field region, the charge density of each oil paper interface was calculated by the ion drift-diffusion model, and the homogenization assignment was carried out. Secondly, for the non-uniform region of the electric field, the value to the non-uniform region should be assigned according to the region of the non-uniform field. According to the interface charge distribution curve of the specific assignment position, the interval was divided and the value was assigned in segments. Finally, the charge field and the applied electric field were superimposed to obtain the whole DC electric field distribution. The experimental verification in the valve-side outlet structure model of converter transformer used shows that the calculated electric field and the measured electric field is pretty close. The method solves the problem that traditional method relys on macroscopic parameters of materials and can not accurately calculate and reflect the real electric field distribution of oil-pressboard insulation system.