Abstract: The reliable operation of longitudinal protection depends on the consistency of channel receiving and transmitting delays, but it is difficult to directly measure or judge the inconsistency of channel receiving and transmitting delays on the engineering site. This article proposes a bidirectional delay measurement method by analyzing the composition and indicator requirements of protection channel delay. To achieve time synchronization between the instruments on both sides during the measurement process, the Beidou/GPS timing system is used as a unified time reference, and a corresponding testing process is designed. This tester significantly reduces the complexity of latency testing, improves work efficiency, reduces manual operation time and costs, and improves the accuracy and reliability of test results.