The synchronization of merging unit in smart substation and limitations of the method of validating whether the current at each side of differential protection is synchronous by using on-site phase and differential current check are analyzed. The design requirements of field testing instrument with merging unit delay characteristics of smart substation is proposed. The system structure and the detailed design of field testing instrument is introduced based on the design requirements. Finally, the detailed calculation and comparative analysis of tester time granularity and the time response with the merging unit is done. Analysis shows that, the tester can validate that the merging unit delay parameters are set correctly, which can prevent incorrect setting and accord with the site acceptance requirements.