Current transformer is one of the important equipment in power system, and is widely used in different voltage levels, its primary side is connected to the primary side of power system, and its secondary side is connected to the secondary system. Therefore it is both primary and secondary equipment, once main insulation fault of current transformer occurs, it will directly affect the reliability of power system operation. In this paper, based on a typical main insulation fault of 500 kV SF₆ current transformer, combined with the disassembly of current transformer, a finite element simulation software named ElecNet of Infdytica is used to calculate and analyze the potential distribution and electric field, the process of fault is explained, it is revealed that air gaps within defective basin insulator cause partial discharge for a long time, which results in main insulation failure. The electric field of current transformer under normal working condition is detailedly analyzed, the fault process is revealed. The research achievements will be of great significance to guide the selection of 500 kV SF₆ current transformer, and improve the operation reliability of power system.