Abstract: As a component of electric energy metering devices, the normal operation of high-voltage current transformers directly affects the fairness of electric energy trade settlement. According to the demand for online error monitoring of high-voltage current transformers, this paper designs an online error monitoring device for 0.05-level current transformers. The design of the internal sampling unit, synchronization unit, computing unit and communication unit is analyzed in detail. Finally, accuracy tests are carried out on the developed prototype. The test results show that the basic error of the online error monitoring device for current transformers designed in this paper meets the 0.05-level design requirements and is suitable for online error monitoring of 0.2-level current transformers.