The invention discloses a step response test system and a method for an all-fiber direct current measuring device. The system comprises a current source, an all-fiber direct current measuring device to be measured and a step response calibrator, wherein the all-fiber direct current measuring device to be measured comprises an optical fiber current sensing ring and a secondary converter; the current source and the optical fiber current sensing ring are connected to form a primary loop, the optical fiber current sensing ring is also respectively connected with a precision shunt and the secondaryconverter, the precision shunt and the secondary converter are both connected with the step response calibrator, and the precision shunt is connected with the step response calibrator through a high-speed acquisition card. The invention also provides a step response test method. According to the invention, the test accuracy of the step response process of the all-fiber direct current measuring device is improved, so that the test result can more accurately reflect the actual performance of the direct current measuring device, and the capability of discriminating the performance of the directcurrent measuring device is effectively improved.