The synchronous sampling performance of a distribution line fault indicator directly affects the expression ability of the synthesized transient zero-sequence current to fault characteristics. This is the basic premise of constructing a reliable fault identification algorithm and has become the core index of equipment quality detection. In this paper, a three-phase synchronization accuracy detection method for a distribution line fault indicator is proposed. First, the time delay between three groups of original sampling sequences is compared by correlation coefficient, and the value range of synchronization error is determined by sliding search. Then combined with a Newton interpolation fitting method, the function of synchronization error and correlation coefficient is constructed to solve the accurate three-phase synchronization time error. This provides an effective and reliable basis for rectifying and adjusting the original recording data. The simulation results show that the proposed method has high detection accuracy, reliable results and effective immunity to the error and noise interference introduced by a fault indicator in the sampling link. This has good engineering application value. This work is supported by the National Key Research and Development Program of China (No. 2018YFB0904500).