Abstract: Space charge is the key factor that affects and restricts the insulation strength of oil-paper insulation used in converter transformer. Compared with the traditional method of restraining space charge by bulk-doped nanoparticles, it is still necessary to explore new idea and method to realize space charge injection. In this paper, the micro/nano structure Al₂O₃ functional film was successfully prepared on the surface of insulation pressboard using the radio frequency（RF） reactive magnetron sputtering method. The space charge and DC breakdown characteristics of the oil-immersed insulation pressboard with micro/nano structure Al₂O₃ functional film were obtained by means of pulsed electro-acoustic（PEA） space charge measurement and DC breakdown measurement under preloading and non-preloading conditions. The results show that the micro/nano structure Al₂O₃ functional film constructed on the surface of insulation pressboard can effectively improve the volume resistivity of the oil impregnated insulation pressboard, inhibit the space charge injection and significantly reduce the electric field distortion in the pressboard caused by the space charge accumulation. These aspects make the preloading DC breakdown voltage of the oil-immersed pressboard with micro/nano structure Al₂O₃ functional film be higher. In addition to the nanoparticle-dependent effect, the micro/nano structure Al₂O₃ functional film enhance the charge injection barrier.