Abstract: In order to study the electrical properties of cellulose pressboard modified by nano-TiO₂ under thermal aging, composite pressboard modified by nano-TiO₂ with different mass fractions and particle diameters were prepared. The two-parameter Weibull distribution model was used to calculate the insulation failure possibility of nanocomposite pressboard, and the relationship between breakdown voltage and doping amount of TiO₂ was analyzed. The insulating pressboards modified by 10 nm TiO₂ with 5% were determined as optimal modified samples. After thermally aging of the pressboard samples, the variation of AC and DC breakdown strength, relative dielectric constant, resistivity, oil immersed pressboard color and surface micro-structure with aging time were obtained. The electric field distributions of AC and DC were calculated by finite element simulation software. The simulation results show that the pressboard has a higher proportion of AC and DC field strength in oil-pressboard nanocomposite insulation. The experimental results show that the AC and DC electrical properties and anti-thermal aging performance of the composite pressboard are always superior to that of the unmodified pressboard under different thermal aging stages. The nano-TiO₂ can weaken the polarization of the cellulose chains, and decrease the carrier mobility. Thus, the internal field strength and electrical performance of the insulating pressboard are improved, and the thermal aging process of oil immersed pressboard insulation is effectively delayed, which are consistent with the simulation results.