Given the field test requirements of step characteristics of a UHVDC measuring device, the field test technology of step response characteristics is studied. A step response test technology of a DC current measuring device based on the BUCK circuit is put forward. A rapidly rising long-pulse large-current source for a step response test of DC current measuring device is developed. The step response test of a UHVDC current measuring devices is completed. The test results show that when the maximum output of the step characteristic calibration system of DC current measurement is 3 000 A, the step square wave rise time is 7 μs, the duration is 10 ms, the overshoot amplitude is less than 1%, and its performance meets the requirements of the detection and verification of a UHVDC current measuring device. This work is supported by Science and Technology of the Headquarters of State Grid Corporation of China (No. GYW17201800033) “DC Measurement Device Test Method Research and Device Development used in DC Project”.