Abstract: Due to the influence of complicated working voltage condition and insulation status of the wiring at the valve side of conversion transformer on the operation reliability of power system, it is useful to study the phase distribution of the needle plate model in the evolution of partial discharge in oil paper insulation under combined ACDC electric field. Based on repeated experiments, both the differences and similarities of phase distribution among different test periods can be found. According to the characteristics of the AC and DC voltage, the mathematical model is built from point of view of the energy conversion, from which such conclusions as the follows can be obtained:the discharge distribution close to 45癮nd 225癷s caused by the AC voltage; The DC voltage strengthens the conversion of electrical energy and potential energy during discharging development, which makes discharging of each phase possible.