Abstract: Moisture is one of the most important factors,which affects the insulation status of oil-paper insulation.The aging of cellulose has obvious effects on the moisture absorption property of insulation paper.In order to estimate the moisture content in insulation paper accurately,Fessler model which is a widely used relation between equilibrium moisture contents of insulation oil and insulation paper,was optimized in this paper by considering the influence of cellulose aging.First,the effects of cellulose aging on the parameters in Fessler model were analyzed.Then,the Fessler model was optimized based on experimental data by considering the effects of cellulose aging.Finally,the optimized equation was testified by experiments.The results show that: in the studied temperature range,the equilibrium moisture content of insulation paper with different aging status can be calculated effectively based on the optimized equation,and it is the basis of further studies and usage in engineering.