Strong electromagnetic interference and some uncertain environmental factors exit when the direct current (DC) transformer is calibrated on the work field. Therefore, the standard DC current comparator should not only have much high measuring accuracy, but also require the strong ability to resist electromagnetic interference. Firstly, the paper establishes the simulation model of the magnetic shielding structure of the direct current comparator (DCC) based on magnetic modulator with the ANSYS software, which is used as the reference for the field calibration. The influence of the thickness of the shielding, magnetic conductivity of the shielding material and gap on the magnetic shielding effectiveness are analyzed. According to the simulation results, the design of the magnetic shielding structure is carried out. Secondly, this paper analyzes the influence of the eccentric bus and interference magnetic field generated by the nearby conductor on the measuring accuracy of the DCC without and with the magnetic shielding structure respectively. In the end, the prototype of the DCC with the nominal parameter of 5000A:5A is developed. The testing results of the prototype confirm the simulation results and the effectiveness of the designed structure. The total accuracy of the developed DCC is up to 0.005 level, which is satisfied with the requirement of the applications.