Online calibration of current transformers meets the requirement of power supply reliability of power system. In view of the fact that magnetic rings of general closed-loop Hall current sensor only have one section of air gap, this paper introduces a kind of which can be used for online calibration. In order to improve accuracy and sensitivity, this paper expounds the design methods from the aspects of selection of magnetic ring material, the size of cross-sectional area of the magnetic ringthe length of air gaps, and the proportion between the total width of two air gaps and the total length of magnetic rings is specifically discussed. The model of open-close magnetic rings is simulated in COMSOL Multiphysics. The experimental results show that the structure mentioned above gives the Hall sensor a wider linear measurement range and better sensitivity. The experimental results provide a theoretical reference for online calibration in the future.