The invention discloses an on-line monitoring method and system for an error state of an electronic voltage transformer. The method comprises: (1), collecting measurement data of a three-phase electronic voltage transformer in calibrated operation, establishing a residual model of the measurement data by using a principal component analysis method, and calculating a control limit of a measurement data statistical feature quantity under the residual model; (2), collecting measurement data of the three-phase electronic transformer in an operating state, calculating a statistical feature quantity based on the residual model established in step (1), and carrying out comparison of the statistical feature quantity with a statistical control limit to determine whether an error state of the electronic transformer is abnormal; and (3), diagnosing error abnormity of the electronic transformer by using a variable contribution rate method when the statistical feature quantity of the measurement data exceeds the statistical control limit. With the on-line monitoring method and system, the traditional periodic etalon detection way can be avoided; and real-time on-line monitoring of the error state of the electronic voltage transformer is realized.