Abstract: This paper introduces the principle of architecture of DC electronic current instrument transformer and the corresponding transient step response character and influence factors. A test method based on the closed-loop verify on the transient step response of DC electronic current instrument transformer is proposed in this paper. Generalized polynomial fitting is used to calculate the step response delay time and other key indicators. The validity of the test method is proved by the newly developed transient step response testing system at the HVDC stations.