This paper studies the electronic current transformer transient transmission delay time from the point view of relay protection, explains the difference between transient state delay time and steady state delay time, as well as the necessity test in engineering. It proposes a test scheme based on simulation sampling with high accuracy and high bandwidth, and the precise time scale digital calibration receiver. Digital phase locked loop (DPLL) is used to eliminate the digital timing jitter. Mutation detecting is applied to determine the initial time, combined with the phase extraction for time delay compensation. The various error factors are eliminated in the test system. The developed test system is applied in the practical engineering, which proves the test scheme is feasible.