In order to solve some problems of the off-line preventive test monitoring of Beijing-Guangzhou high-speed rail traction substation 220 kV dry type current transformer, and on the basis of the insulation dielectric loss of physical model, this paper uses the zero monitoring comparison method and designs a set of 220 kV dry type current transformer real-time online monitoring device. In the design process, it carefully analyzes influence on the tan delta value measurement precision caused by current sensor, high order harmonic, zero-crossing comparator offset voltage, zero drift, environmental temperature and humidity, etc. According to the different factors influencing the accuracy, the corresponding technical improvement measures are put forward. Field operation shows that the tan delta value results of real-time online monitoring accord with that of offline measurements, it has stable running and reliable work, and can meet the requirements of field operation.