The establishment of oil-paper insulation composite system health state assessment model is of great significance for the protection of transformer safety, for the current oil-paper insulation state assessment model of a single characteristic quantity and state assessment method is too abstract and so on, this paper is based on the partial discharge maximum discharge per unit, average discharge per unit and discharge pulses number per unit put forward a three-element mixed insulation oil-paper system health state assessment model. Through the partial discharge experiment to obtain the basic characteristic parameters affecting the partial discharge of oilpaper insulation composite system, its comprehensive analysis and processing, and finally through the radar diagram to establish a three-element mixed insulation oil-paper system health state assessment model. The assessment model has different areas under different health states of the oil-paper insulation system, which solves the problems of empty and overlapping areas of the traditional assessment model, and is sufficient to respond accurately and effectively to the health state of the oil-paper insulation at different stages, and provides guidance for formulating overhaul strategies for the three-element mixed insulation oil transformer.