The accuracy and stability of electronic transformers (ETs) is the guarantee of the safe and reliable operation of a power system. As an emerging technology, however, the ETs’ failure rate is higher than that of traditional transformers. As a result, the calibration period should be shortened. Existing calibration methods address the verification on condition that the ETs and lines are powered off, which has a long calibration period and results in a power-off loss. As a result, an online calibration system for electronic voltage transformers (EVTs) is proposed in this paper. Employing a specially designed standard voltage online measuring unit, the proposed system can obtain the high voltage signals without powering off and do the calibration when the EVTs are working. The power-off loss can be avoided, and problems of the running EVTs can be detected in time. The main factors influencing the system accuracy are addressed. The system has been validated by the Quality Inspection and Testing Center of the China Electric Power Research Institute, and field tests have been conducted in Guizhou Electric Power Company. The results show that the proposed system has good reliability and can reach an accuracy class of 0.05 in complicated electromagnetic circumstances.