The present invention discloses a transformer insulation paper aging diagnosis method based on a characteristic of a dispersion dyeing image of fibers in oil. The method comprises a step of using a dispersion dyeing method to obtain a dispersion dyeing image of suspension fibers in the oil, a step of establishing an association relation model between the dispersion dyeing image and an aging degree, a step of obtaining a reference point set of a non-aging dispersion dyeing image of the suspension fibers in oil, a step of calculating Mahalanobis distances MD between dispersion dyeing images of the suspension fibers in the oil and the reference point set in different aging states, and a step of judging the aging state of insulation paper according to a degree of polymerization of DP. The invention provides the diagnosis method, by using the dispersion dyeing images of the suspension fibers in the oil of different aging stages observed by using a dispersion dyeing method, the change rule of the dispersion dyeing images with an aging time is obtained, and the association relation between the aging state of the insulation paper and the dispersion dyeing image characteristic is established. The method is a direct measurement method and has the advantages of high accuracy, a simple operation, no influence of operations of oil change and the like, and a large application prospect.