Condition monitoring of transformer oil plays an important role in transformer maintenance. The insulation performance of transformer oil mainly depends on its breakdown voltage which is greatly affected by the moisture quantity in transformer oil. In order to overcome the shortcomings of the existing detection methods that are susceptible to electromagnetic interference

this paper proposes a detection method based on Micro-nano fiber（MNF） evanescent field sensing. The method is based on the principle that the change of moisture quantity causes the change of refractive index

affecting the distribution of evanescent field of the MNF

thereby resulting in the change of output optical power. Then

an MNF fusion taper platform was built up to prepare MNF with the diameter of 10～125 μm. The experimental results show that the diameter of MNF is an important factor which affected the moisture quantity detection in transformer oil. As the diameter of MNF decreases

the sensitivity increases. When the diameter reaches 10 μm

the sensitivity of the moisture quantity detection is higher than 5 mg/L

therefore it meets the requirements of field oil immersed power transformers. The MNF sensor can achieve real-time measurement of moisture quantity detection in transformer oil and can be used for on-line monitoring of oil-immersed power transformers.