Aiming at the online measurement of dielectric constant of lubricating oil mainly relies on imported sensors, a dielectric constant sensor based on CDC microcapacitance change detection and concentric cylindrical ring electrode is designed. The detection mechanism of oil dielectric constant sensor is expounded, the structure and parameters of concentric cylindrical ring detection probe are designed, and the hardware circuit and software communication program for the micro-capacitance change of lubricating oil based on CDC digital capacitance conversion chip are developed. After calibrating the sensor, the results of the retest and the oil dynamic test under different water content conditions show that the sensor has high sensitivity and good repeatability, reflect the dielectric constant characteristics of lubricating oil with different water content, and has good dynamic response and stability. In the engineering application of a wind turbine gearbox, the maximum deviation of the sensor is 2%, and the data stability is well, which can meet the needs of the sensor oil online monitoring field application.