Compared with the traditional substation, the delay uncertainty in the secondary circuit of the intelligent substations increases, which greatly affects relay protection. In order to measure the whole delay characteristic of the secondary circuit and detect the single device, first of all, this paper analyses the delay link in the secondary circuit of the intelligent substation. Then, according to the form of information transmission in the secondary loop, the detection method and experimental scheme for the delay of the whole loop are put forward. Finally, the measurement results of a group of test instruments using the experimental method are given. The measuring method and the experimental scheme have the advantages of high measurement precision, simple operation and easy implementation. It can measure the delay of the single device and the whole delay of the loop in the secondary circuit, so that the staff can grasp the operation and maintenance of the intelligent substation from the whole situation. It has certain guiding significance for the operation and maintenance of the device in the secondary circuit. This work is supported by Science and Technology Project of State Grid Corporation of China (No. 52094015001A).