Oil-paper insulation is the main insulation form of oil-immersed transformer, whose properties declining affects the safe operation of transformer. Therefore, it is important to evaluate the insulation aging of transformer. During transformer operation, furfural produced by aging of insulation paper diffuses into the oil ceaselessly, so the furfural content in oil is considered a valid way to assess insulation aging. In previous thermal aging experiments, it was found that moisture would affect the distribution of furfural between oil-paper system mightily, yet the mechanism of influence and the law of diffusion equilibrium were not clear, therefore corresponding solution was not available. To elucidate the above problem, molecular dynamics simulation and equilibrium test of furfural diffusion process at typical operating temperature of transformer were carried out in this paper to explore the law of moisture influence on furfural diffusion equilibrium. The results show that furfural will diffuse into oil due to the increase of moisture content in paper, which attributed to the increasing moisture content reduces the attraction of paper to furfural, enlarges the space of inter cellulose and expands the diffusion space of furfural. When the moisture content in paper increases from 0.5% to 1%, the furfural content in oil will increase correspondingly under different aging conditions, so the modified furfural analysis model based on moisture equilibrium curve in oil-paper insulation is constructed for the aging assessment of oil-paper system with different moisture content.