Abstract: The chromogenic characteristics of fibers in oil can indicate the aging state of paper insulation. Research indicates that the average refractive index of aging fibers in oil rises, but the underlying mechanism remains unclear. To study the mechanism of increasing refractive index of fibers in oil during thermal aging, based on the theory of cellulose two-phase system, the change characteristics of crystalline and amorphous regions in cellulose aggregation state during thermal aging process were studied, and the change rule of cellulose crystallinity with the aging time as well as the correlation relationship between the refractive index of cellulose and the crystallinity were analyzed. Based on these, a cellulose amorphous zone model was built to analyze the changing rule of the ordered degree of cellulose amorphous zone in the process of thermal aging. The results show that the recrystallization of the cellulose amorphous zone during thermal aging leads to an increase in the crystallinity of cellulose. The refractive index of cellulose is positively correlated with its crystallinity, and the theoretical calculation of the refractive index of cellulose can be in good agreement with the experimental test value. Simulations show that the degree of ordering of the cellulose amorphous zone increases with the increase of aging time, which further verifies the recrystallization phenomenon of the cellulose amorphous zone and reveals the mechanism of the increase of the refractive index of the fibers in oil during the thermal aging process. This study provides a theoretical basis for the subsequent assessment of the thermal aging state of paper insulation based on the chromogenic characteristics of fibers in oil.