Capacitor voltage transformer，the important primary equipment of power system，transfers the high voltage into standard secondary voltage and provides voltage information for the protection，metering and measurement device. In view of abnormal heating of 500kV capacitor voltage transformer in this paper，the reason of the problem is analyzed and found by infrared temperature measurement before power off and the capacitance，dielectric loss and insulating oil test after the power off that the reason is the discharge after virtual connection between the internal connection piece and the connection position of the upper cover. The connection piece falls off under the joint action of the heat and electrical corrosion and bounds to the expander. The long term discharge causes breakdown of expander and the insulating oil leaks from the upper cover flange. At the same time，the heat generated by the discharge is transferred to the flange，leading to abnormal heat. Finally，in view of such issue，the corresponding preventive measures are proposed so to assure safe and reliable operation of equipment.