As an important part of the UHV energy metering device, Capacitor Voltage Transformer (CVT) needs periodic error checking to determine whether it meets the accuracy level requirement. At present, there are several problems needed to be solved in CVT error checking:(1) It can not carry out the full voltage acceptance test on the CVT beside the high resistance; (2) Off-line calibration is hard to implement; (3) The verification result deviates from the actual value of CVT online state, so it is urgent to carry out on-line peer comparison research. Under this background, this paper takes the error characteristics of UHV CVT as the research object and studies the research device and test method of on-line error checking.The developed online checking system is used in error field test of UHV capacitor voltage transformer of Zhejiang-Fuzhou UHV project and the on-line error characteristics data during CVT operation is obtained, verifying that using the method can achieve CVT on-line peer comparison. This work is supported by Science and Technology Project of State Grid Corporation of China (No. GYB17201600161).