Abstract: As the aging state of oil paper insulation structure can basically reflect the whole aging state of transformers,it is of great significance to study the aging state evaluation of oil paper insulation for the prediction of a transformer’s life.However,there are few studies on the gas production status of thermally upgraded paper and vegetable insulating oil applied to oil-paper composite insulation systems at home and abroad.Nomex paper-mineral oil and Nomex paper-vegetable oil insulation samples are respectively subjected to accelerated thermal aging tests and the gas production laws and gas composition of the two kinds of composite samples during thermal aging process are studied.The results show that the gas production of Nomex insulation paper in vegetable insulating oil is significantly higher than that in mineral insulating oil on the same thermal aging condition.And the most characteristic gas produced in mineral insulating oil is CH₄ while that produced in vegetable insulating oil is C₂H₆.The insulation status of Nomex paper-mineral oil and Nomex paper-vegetable oil is diagnosed and judged with DGA three-ratio method.It is pointed out that the identification results of Nomex insulating paper-mineral oil are consistent with the actual situation while the results of Nomex paper-vegetable oil are not consistent with the actual situation,which provides certain reference for the diagnosis of transformers with the new type of oil-paper insulation system in the future.