The problem of oil-paper insulation partial discharge has attracted widespread attention in the industry. In order to explore the characteristics of partial discharge under AC-DC composite voltage, this paper built a measurement platform during the research process of this problem, and designed air gap discharge, suspension discharge, and cylindrical surface discharge. Discharge models of four types of typical defects of oil-paper insulation. Combining the experimental results of these four kinds of partial discharges, after analysis, it is found that the partial discharges of the four typical defects will change differently under different composite voltage ratios, and the average discharge amount and DC component of the first three defects at the initial stage are negative. The relationship between the characteristic of forward change and the initial voltage of partial discharge is a positive change characteristic. Compared with the other three typical defects, the initial voltage of partial discharge in the suspended insulation discharge model under AC and DC mixed voltage is obviously increased, and the average discharge under AC voltage is reduced. The results obtained provide basic data for the use of partial discharge tests to determine the effect of the type of commutation transformer defect and the voltage form on the partial discharge.