In digital substations with electronic transformers， the key for implementation of line differential protection is the synchronization of electrical quantities at both two sides. A new method is proposed to solve the problem. First， merging unit（MU） and protection device of digital stations are synchronized by unified clock source. Then data of ac quantities used for protection are calculated with interpolation method. Finally， protection devices synchronization in substations at two sides are completed by sampling time adjustment method. And the synchronization of electrical quantities at both sides is achieved.