Abstract: Space/interface charge is a key factor affecting the insulation performance of valve side outlet device of converter transformer. Studying and understanding the interface charge characteristics of "liquid-solid" oil paper insulation under AC and AC/DC composite voltage will contribute to the development of UHV transmission system in China. By means of numerical simulation, this paper studies the interface charge characteristics of "liquid-solid" oil paper insulation under AC and AC/DC composite voltages. Firstly, the simulation method of space/interface charge of "liquid-solid" oil paper insulation system under AC and AC/DC composite voltages is studied. Then, focusing on the dynamic change of space/interface charge with phase under AC voltage, the simulation analysis and deduction calculation model of space/interface charge distribution of "liquid-solid" oil paper insulation system under AC and AC/DC composite voltage are carried out. The research results show that, for multi-layer "liquid-solid" system, the error between the simulation result and the experimental result is less than 1%. The interface charge density increases exponentially with the increase of field strength and temperature; the steady-state interface charge density increases with the increase of AC/DC composite ratio (the ratio of DC voltage to AC amplitude) and gradually reaches a stable value. On the basis of previous study results, a model to calculate the interface charge of multi-layer "liquid-solid" oil paper insulation system under AC and AC/DC composite voltages is established.