Abstract: The whole system testing is a key item for checking the sampling synchronization of relay protection equipment and the wiring correctness of loop. The problems of whole system synchronization testing caused by distributed sampling in smart substation is firstly analyzed in this paper. And then the advantages, disadvantages and adaptability of the two conventional testing methods are compared and discussed. A new testing method and its function design are proposed based on the total station GPS synchronous signal extension device. The testing results show that this method has high precision and good adaptability.