The concept of group online error calibration for voltage transformer is proposed for these problems of primary equipment condition-based maintenance. First, the choice of voltage standards for group online error calibration is introduced base on the actual operation of substation. Second, the principle and structure of group online error calibration is illustrated. What’s more? Two groups of test for multi-channel signal acquisition at the same time have been done for the online calibration system which is developing in the paper. The experimental data show that the error which is produced by the calibration system is less than 0.05% at run time and it can meet the ability to 0.2 level calibration voltage transformer. Finally, in order to improve the reliability of online calibration, provisions for firstly and secondary voltage drop, second load are carry out accord characteristics of field test.