This paper investigates the charge distribution and electric field characteristics of oil-paper insulation under a AC/DC composite electric field. The existing resistances-capacitance model and finite element simulation methods have problems such as nonlinear electrical conductivity of oil-paper and inaccurate calculation of interface charge density. His paper proposes a method for calculating the AC/DC composite electric field of oil-paper insulation considering interface charges through COMSOL simulation. Another calculation method is also proposed in this paper. When calculating the AC electric field, the influence of interface charge and trapped charge is considered, and a coefficient less than 1 varying with the electric field is introduced to adjust the conductivity of the insulating paperboard to eliminate the error caused by ignoring the trapped charge. The paper provides two methods for calculating the composite electric field, and the calculation results are closer to the actual measured values and have smaller errors compared to the traditional RC model. This research provides a new approach for further analysis of the electric field characteristics of the oil-paper insulation structure, which can guide the evaluation of insulation performance and the optimization design of high-voltage equipment.