The distributed sampling value and its synchronous mode in smart substation are firstly analyzed. Then the advantages, disadvantages and adaptability of the two conventional testing methods are discussed. A new testing method and its function design are proposed based on total station synchronous signal extension device. The synchronous signal which has been laid out in switch yard is used to be the testing reference in this method. Finally, the precision of this method is tested. The results show that the method has high precision and good adaptability.