In this paper，a fault of abnormal voltage reduction at the secondary side of capacitor voltage transformer（CVT）on the high voltage side of 35 kV main transformer in Yunnan is introduced. The cause of the fault is analyzed in combination with the structure and operating principle of CVT，test data and the internal situation of the equipment after disassembly，and also by using LT spice circuit simulation software and formula derivation. It is judged that the fault is caused by the breakdown of the shunt zinc oxide arrester in the electromagnetic unit of CVT due to the aging of the valve plate，resulting in the short circuit of the compensation reactor，and the inductance component of the primary side of the electromagnetic unit is decreased greatly，resulting in variation of the primary voltage of the electromagnetic unit，thus causing the abnormality of the secondary voltage. This analysis provides an important experience reference for similar troubleshooting of CVT equipment in the future.