Abstract: The lower the dielectric constant of paperboard in oil paper composite insulation under AC voltage is, the higher the overall withstand voltage level will be. Different preparation process parameters of paperboard will directly affect the dielectric constant and breakdown field strength of insulating paperboard, whereas, there is a lack of collaborative optimization between preparation parameters, such as beating and hot pressing process, and dielectric and breakdown properties. In this paper, starting with researching the beating time, beating pressure, hot pressing time, hot pressing pressure, hot pressing temperature, and other processes, the influence relationship and weight of the preparation process on the properties of 1 mm high-density softwood insulated paperboard were studied. By changing the combination of different process parameters through the weight, the purpose of reducing the dielectric constant of oil-immersed paperboard while keeping the breakdown performance unchanged was achieved. The internal space charge of oil immersed paperboard was measured by the electroacoustic pulse method, and the surface trap parameters of oil immersed paperboard were measured by the surface potential attenuation method. The influence mechanism of process and surface structure on properties was revealed. The results show that, for the purpose of matching dielectric and breakdown properties, the best process of high-density softwood insulating paperboard is as follows: the beating parameters are 3 kg and 60 min, and the hot pressing parameters are 160 ℃, 50 min and 15 MPa. The high beating pressure makes the fiber more broomed, which limits the charge transmission between the fibers and improves the breakdown field strength of the oil impregnated paperboard. Appropriate hot pressing temperature, hot pressing pressure and surface structure can reduce the density and depth of traps on the interface and surface, and make the charge distribution more uniform, so as to reduce the interface polarization and dielectric constant. The results provide guidance for the preparation and performance control of insulating paperboard.