Pressboard samples with different aging degrees were prepared to study the material and discharge characteristics of pressboards with a shielding electrode. The step-up method was used for the subsequent experiment. The partial discharges of the pressboards with different aging degrees were compared and combined using Fourier transform infrared spectra, as well as electric field simulations and analysis. The microstructure of the aged pressboards was observed with scanning electron microscopy. The results showed that pre-discharge occurred mainly in the oil gap between the hemispherical electrode edge and pressboard. Aging the pressboard had no obvious effect on the discharge characteristics at the initial partial discharge stage. As the partial discharge developed, aging the pressboard resulted in a higher electric field intensity acting over a greater area, which led to a more intense discharge. Microscopic observation showed that aging and discharge destroyed the fibre, reduced the fibre width, and caused holes and fractures, which promoted further development of the discharge. The spectroscopic analysis showed that aging destroyed O-H functional groups and reduced the intermolecular forces and mechanical properties of the pressboard.