The major difficulties encountered during the data synchronization process of optical fiber differential protection connected with electronic transformers（ET） are pointed out， and the shortcomings of GPS and traditional data synchronization methods are analyzed. In addition， a new data synchronization method employed with improved inserting is proposed. The method depends not on any peripheral equipment and has high reliability. It doesn't need to adjust the sampling time in compliance with the function and frame condition of merged unit（MU） specified in standard IEC 60044-8，and it can be applied flexibly to the case that one side connected with ET and other side with traditional transformer. This paper shows that the target of data synchronization is to ensure that voltage and current value in the differential calculation are simultaneously traced back to primary side. Therefore， the secondary transform delay of ET at every side must be accounted in protection data synchronization process. As for the protection， compensation of time difference of the secondary transform delay at two sides of ET is the core content of data synchronization. For the line， which needs to use optical fiber differential protection， the MU of the ET may provide better data output interface according with standard IEC 60044-8.