There is a high demand for data synchronization in the sampling process in electric power system. Synchronization method is recommended by the interpolation algorithm in IEC61850 communication protocol. Here this paper puts forward an improved Lagrange interpolation algorithm. By using the method of constructing weighting function, it can compensate the error of the first-order interpolation algorithm and can improve the computational efficiency. Finally, the simulation experiments and a comparative analysis are carried out for the improved algorithm under different conditions such as steady-state characteristic for different sampling frequency, the transient characteristics of decaying DC component, limit of voltage sag, etc. Experiment result shows that the improved algorithm has many obvious advantages in computation and accuracy.