Abstract: Accurate in-situ detection of dissolved furfural content in oil is very important for evaluating the aging degree of oil-immersed transformer paper insulation, but the existing in-situ detection methods have the problem of low detection sensitivity. In view of the above problem, this paper proposes a method to improve the Raman signal of furfural dissolved in oil by modifying 4—aminothiophenol (4—ATP) molecules on the surface of B-Cu@F-Ag@Au substrate. First, the Raman signal enhancement effect of 4—ATP—furfural is verified and analyzed based on simulation methods. Then, the standard furfural insulating oil sample and aging oil sample are detected by preparing 4—ATP—B-Cu@F-Ag@Au substrate, and the high-sensitivity in-situ detection performance of the proposed method for furfural in insulating oil is verified. Finally, the detection accuracy of the proposed method is clarified by comparing with the results of liquid chromatography (LC). The results show that the introduction of 4—ATP molecules makes the C＝O bond of furfural with weak Raman signal intensity Schiff react into a strong C＝N bond, and the minimum detection limit of furfural dissolved without extraction is reduced by 25 times, reaching 0.08 mg/L. Compared with the results of LC, the absolute deviation and relative deviation of in-situ detection of dissolved furfural in laboratory accelerated aging and on-site operation transformer oil samples (No.1, 2 and 3 transformers of Chongqing Changshou 500 kV substation) are less than 0.005 5 mg/L and 4.18%, respectively. It is proved that the proposed method can realize the highly sensitive in-situ detection of dissolved furfural in mineral insulating oil without extraction, which can provide technical support for ensuring the safe and reliable operation of oil-immersed transformers.