Abstract: Trap plays an important role in the transportation of charge and the investigation of trap parameters is significant to the further research of discharge mechanism. According to the turn-to-turn structure of on-board transformer, a three-layer oil-paper insulation defect model was designed for partial discharge experiments. The oil-impregnated paper from the beginning to the breakdown of partial discharge was selected to measure the surface potential attenuation curves of different layers of insulating paper in three layers of oil paper insulation medium by isothermal surface potential decay. The trap distribution of insulation paper in different stages of partial discharge (PD) was calculated and its influence on discharge characteristics was studied. The results show that the deep trap is the majority of Nomex paper, and the trap energy of the first layer increases with the development of partial discharge but the energy of the second and the third layers shows an opposite trend. The change rate of free charge increases with the decrease of trap density and trap energy, and the acute discharge increases with the increase of free charges. The transmission characteristic of charge contributes to the different evolution of the trapping parameters of different layers.