A protection current circuit defect, which is caused by the short connection of a discharge gap metal sheet in the secondary junction box of a current transformer, is analyzed in this paper. Based on the protection current circuit diagram, the equivalent circuit model is presented. At the same time, the current shunt of 500 kV bus-bar differential protection in a quasi-steady state, caused by the short connection of a discharge gap metal sheet, is analyzed theoretically. On this basis, combined with the simplified equivalent circuit model, a circuit simulation model is built by the Pspice. Then, the influence of this anomaly on the sampling current of a 500 kV bus-bar differential protection device is further highlighted. Finally, combined with the protection logic of 500 kV bus-bar differential protection, the influence of a discharge gap anomaly on 500 kV bus-bar differential protection function is analyzed in a quasi-steady state. This work is supported by National Natural Science Foundation of China (No. 51807028).