The necessary of error test for 1000 kV CVT under operation condition is analyzed. The factors influencing the CVT error such as temperature variation, frequency, adjacent charged body and the secondary load are discussed. The wiring difference of routine test, on-site acceptance test and online operation of 1000 kV CVT is compared. On this basis, the principle and the method of error measurement for 1000 kV CVT when operation are proposed and a 1000 kV CVT online error calibration system is developed. Appling the calibration system, error tests for 1000 kV CVT outlet side in engineering operation are finished. The error test provided is significant to the design, construction, on-site acceptance test, and online monitoring of subsequent UHV project. This work is supported by Science and Technology Project of State Grid Corporation of China (No. GYB17201600161).