To study the aging effects of pressboards on the development law of oil-paper insulation surface discharge, various aged pressboards were made up by thermal aging method, and the plane-plate model was designed. The partial discharge signal were measured and stored by constant-voltage method for long time experiment, and the total discharge quantity, the discharge number and the maximum discharge quantity are statisticed and the influence of pressboard aging on the surface discharge development law is summarized as well. The cause of this influence is analyzed based on the theoretical and simulation analysis. From the experimental result, it is concluded that the discharge occurs in the high field strength region of transformer oil in the early discharge stage, and the pressboard aging has no obvious impact on the surface discharge development law. However, with the development of discharge, the transformer oil in the pressboard pore which is caused by the aging effect is decomposed to produce gas, which will increase discharge degree. The pressboard aging has great impact on the surface discharge development law in the development and late stages: the higher the degree of aging, the more rapid increase and lager value of the total discharge quantity, discharge number, and the maximum discharge quantity.