The harmonic transfer characteristics of electromagnetic current transformer is complex, and the transfer error varies and is hard to estimate. This article designs the experiment of hysteresis loop of ferromagnetic material, and analyzes the influence of the hysteresis loop shape on the transfer error of current transformer. Aiming at the difficulty of high accurate detection of harmonic error, a highprecision detection system of current transformer harmonic error was developed. With the experiment, the influence of frequency, load, fundamental current, harmonic phase and DC component on the harmonic transfer error of current transformer is analyzed.