The delay time of sending SV packets by RTDS-GTNET network protocol card is inconsistent with the configured delay time of Timedelay delay channel. When it is applied to the closed-loop test of digital relay protection device, it will cause errors, even affecting the correctness of the test result. In view of this situation, this paper presents the testing technique and scheme of the delay characteristic of the RTDS-GTNET packing SV packet, and carries on the statistical analysis of the sample with the common methods. The simulation verifies that the test scheme is correct, it can obtain the exact delay time of the GTNET packing SV packet through test, which provides an accurate method for testing the data sampling process and the logic correctness of the relay protection device more accurately during the practical applications. This work is supported by Science and Technology Program of State Grid Corporation of China (Critical technology and platform construction of multi-energy system hybrid simulation) and Science and Technology Program of State Grid Zhejiang Electric Power Research Institute (No. 5211DS15002V).