Abstract: To overcome the problems of existing calibration methods for electronic transformers（ETs）,such as the need of power off and complicated operation,an on-line calibration system for electronic voltage transformers（EVTs）with digital output is proposed.By employing a rhombic hoist,the standard voltage transformer can be connected to the high voltage bus,and then a standard voltage on-line measuring unit that can measure the high voltage signal on-line is constructed,realizing online calibration of the running voltage transformer.In this paper,the factors affecting system accuracy,such as the synchronization methods and the software algorithm,are analyzed in detail,and measures to reduce errors are taken.The system developed has been certified and applied on the spot,the accuracy better than grade 0.05.