An improved interpolation method is proposed， which can solve perfectly the data synchronization problem at two sides of the line while the merged unit（MU） of differential protection connects according to IEC 61850-9 standard interface. It doesn't need to adjust the sampling time in compliance with the function and frame condition of MU specified in electronic transformer（ET） standard， and it can be applied flexibly to the case that one side connected with ET and the other side with traditional transformer. The data synchronization of the method completely gets rid of global positioning system（GPS） recommended connotatively by the standard and improves protection's reliability greatly. In addition， the logic and application methods that MU transmits or self-produces the 1PPS signal shared by protection and MU are proposed in the paper. The method further improves the reliability of the protection and ensures the protective function independent of any other devices except MU and protection itself.