Abstract 2-furfuraldehyde (2-FAL) is widely accepted as a chemical marker to indicate the degradation of cellulose insulation in oil-impregnated transformers. However, the application of 2-FAL in the aging diagnosis of transformers in operation is ineffective. The excessive dispersion of 2-FAL concentration measured in transformers is caused by many influence factors. In addition, the 2-FAL concentration in oil cannot indicate the aging of the most serious point given the temperature distribution along the winding height, that is, the insulation at the hot spot of a winding. To optimize the assessment, an improved method for the hot-spot insulation assessment based on the 2-FAL concentration in oil is proposed in this work. The 2-FAL generation in cellulose chain scission is firstly analyzed theoretically, and a new linear relationship between ( 1 / D P t - 1 / D P 0 ) and 2-FAL total generation is obtained. Moreover, the 2-FAL partitioning between an insulation paper and mineral oil influenced by temperature, moisture, and aging status in total is considered. Second, by considering the linear relationship and the 2-FAL partitioning, an equation between the insulation at hot-spot temperature and the 2-FAL concentration in oil is established for the thermal aging under unequal temperature distribution condition. Finally, to verify the proposed method, a testing platform is introduced to conduct the thermal aging experiment with temperature gradients. The proposed method demonstrates better assessment results than the conventional thermal aging test at constant temperature.