Abstract: The performance of the oil-paper (OP) insulation directly determines the operating life of the converter transformer. The investigation of space charge behavior in OP insulation under composite electric field is the key to analyze the electric field distribution and improve the insulation reliability. In this paper, space charge distribution is measured under the DC, AC and composite fields in the OP specimens with different aging degrees. By comparing the space charge distribution characteristics of composite field and single field, the role of different types of electric fields in the process of charge transport is analyzed. On this basis, the charge accumulation mechanism under the AC field is studied with the consideration of the non-uniformity of OP insulation. The results show that the periodic reversal of the electric field leads to a charge hysteresis phenomenon under the AC field, and the difference of material properties leads to the different contribution of charge injection and charge hysteresis in the space charge distribution. The charge accumulation under the AC field varies with the frequency of the applied field, which is essentially the performance of the carriers with different mobility selected by the frequency. The effects of directional migration under the DC component and charge hysteresis under the AC component are both reflected in the charge distribution under the composite field. The increase of the frequency promotes the charge accumulation under composite field. Exploring the characteristics of charge accumulation under composite field provides support for the further study on the insulation degradation under working conditions.