At present, there is not much research on using dielectric constant to characterize the Acid number (AN) of transformer oil. However, some studies are exploring how to use the dielectric constant to monitor and evaluate the quality and performance of transformer oil. This study developed an AN online monitoring sensor based on the changes in dielectric constant caused by the products of aldehydes, ketones, carboxylic acids, and hydroxy acids in the induction, development, and hysteresis phases of transformer oil AN thermal oxygen aging. The transformer oil with different ANs ranging from 0 to 0.26mgKOH/g was tested, and the lowest repeatability of the sensor was 0.33%. Compared with the offline AN automatic potentiometric titration instrument, the lowest reproducibility was 4.54%. This provides a basis for characterizing the dielectric properties of transformer oil with subsequent dielectric constant characterization.